hello everyone! how are they? welcome once again to my youtube channel this time we are going to continue talking about the embryonic period but this time we are going to develop the external morphological changes that the embryo goes through during this stage if you missed the video of the embryonic period I will leave it to you here on the top right as a card so that you can access it more quickly and easily during the embryonic stage the age of the embryo at first is expressed based on the somites and later when the somites count begins to become difficult Age is related to the cephalocaudal length of the embryo and is expressed in millimeters.

This cephalocaudal length corresponds to the measurement between the vertex of the skull and the midpoint between the apices of the embryo's buttocks, as we saw in the previous video that in the embryonic period the differentiation of the embryonic layers took place and the formation of the organs and systems in this stage also occur great After the changes in the external appearance of the embryo, at the end of the third week, between days 20 and 21, the first three pairs of somites appear and the embryo acquires its final tubular shape, measuring between 1. 5 and 3 millimeters in length on the other hand in the The dorsal surface shows the neural groove limited laterally by the neural folds in the fourth week, on days 22 and 23, the embryo has between 4 and 12 pairs of somites and measures between 2 and 3. 5 millimeters in length, and begins to curve due to the growth of its cephalic and caudal ends and the first two pharyngeal arches appear, insinuating between them the stomodeum between days 24 and 25 there are between 12 and 20 pairs of somites and the length is between 2.

5 and 4. 5 millimeters it is accentuated Embryo incubation and optic vesicles and otic placodes can be seen on either side of the cephalic end of the embryo. The first pharyngeal arch becomes prominent and the maxillary and mandibular processes are seen in it.

lar limiting to the stomodeum that is covered by the oropharyngeal membrane. Finally, the rostral and caudal neuropores are still open but are very small on days 26 and 27. The embryo is already made up of 21 to 29 pairs of somites and reaches a length of between 3 to 5 millimeters likewise the embryo begins to adopt a "C" shape due to the development of the cerebral vesicles and from its caudal end the rostral neuropore will be closed and the optic vesicles are evident the oropharyngeal membrane begins to perforate and appears the third pharyngeal arch the otic plates sink and become the otic fossae and finally the buds of the upper limbs appear during the fifth week the embryo grows considerably between days 28 and 30 there are between 30 and 35 pairs of somites and their length is between 4 and 6 millimeters at the level of the optic vesicle the lens plates are formed the otic fossae deepen and become the s otic vesicles on the sides of the stomodeum the olfactory plates can be seen and the fourth pharyngeal arches appear the upper limbs adopt the shape of a fin and the buds of the lower limbs appear also the caudal neuropore closes and its caudal end begins to attenuate between on days 31 and 32 the somites continue to form but no longer protrude from the surface, making it impossible to count them the length of the embryo is 5 to 7 millimeters its head is flexed on the trunk and the cervical curvature is observed the optic vesicles are they become the optic cups and the lens plates unite to give rise to the lens vesicles likewise the olfactory plates become the olfactory pits the upper limbs take the shape of a paddle or oar and the lower limbs that of a fin Yes We observe this embryo under the microscope, we will see that by transparency we can visualize the primordial cerebral vesicles.

arias and pontine curvature between days 33 and 36 the embryo measures between 7 and 9 millimeters in length the primary brain vesicles transform into the secondary brain vesicles and the lens vesicles separate from the ectodermal surface the olfactory pits deepen and their lateral edges grow , forming the medial and lateral nasal processes , in addition, the second pharyngeal arch grows until it hides the third and fourth arch and gives rise to the cervical sinus and finally the upper limbs flatten at their distal end forming the plate of the hand while the lower limbs adopt the shape of a shovel or oar during the sixth week radical changes are observed in the facial features of the embryo between days 37 to 40 the embryo reaches a length of 8 to 11 millimeters the auricular prominences appear on the edges of the first pharyngeal groove and the eyes appear a dark coloration given by the pigmentation of the retina in some embryos can be distinguished in the limbs upper rivers the buds of the elbow and wrist and in the lower limbs the foot plate begins to form on days 41 to 43 the length is 11 to 14 millimeters the cerebral vesicles grow considerably the segment of the neck and the head begin to be distinguished to straighten with respect to the trunk, the auricular prominences are more prominent , as well as the elbow and wrist of the upper limbs , and finally, on the plate of the hand, the digital rays appear that will form the outlines of the future fingers in the seventh week of the embryo. it increases considerably in all its dimensions by days 44 to 46 the embryo is between 13 and 17 millimeters in length the buds of the eyelids appear and the auricular prominences fuse to form rudimentary auricular pavilions the cervical sinus is obliterated by the fusion of the second pharyngeal arch with lateral neck wall also becomes visible nipples on ventral thorax wall the digital rays of the hand plate are evident and some notches appear on the free edge and finally the digital radiations are observed on the foot plate on days 47 and 48 the cephalocaudal length is 16 to 18 millimeters the trunk region it lengthens and straightens and the upper and lower limbs increase in length approaching the midline the fingers of the hands begin to become free and inside the base of the umbilical cord the development of the intestinal loops begins to be appreciated on days 49 to 51 the embryo reaches a length of 18 to 22 millimeters in the head the vascular plexus of the scalp can be distinguished, the eyelids and the auricles are also more noticeable in the hands the fingers are widely separated and in the lower limbs they can be distinguished the knees and indentations in the free edge of the foot plate the intestinal loops bulge the base of the umbilical cord considerably and are also It can be noted that the tail of the embryo is very short but still visible these days at week 8 the embryo concludes its embryonic stage on days 52 and 53 it reaches a length of between 22 to 24 millimeters its head begins to round and its neck becomes the hands and feet are also straightened and lengthened, they are brought closer to their contralateral side, on the one hand the fingers of the hands are completely separated and on the other the toes are identifiable although they remain united and these days the toes are also visible intestines inside the umbilical cord on days 54 and 55 the length of the embryo is 23 to 28 millimeters the neck region is constituted and the eyelids are highly developed almost completely covering the eyes the auricles begin their ascent from the region of the neck and the toes of the feet are freed, disappearing between them the interdigital membrane that joined them, the heel is also distinguishable, allowing nding the delimitation of the foot and its quantification on day 56 the embryonic stage ends and the fetal stage begins the cephalo-caudal length of the embryo is 27 to 31 millimeters its head is rounded and its size decreases proportionally with respect to its body the eyelids are completely covering the eyeballs and begin the epithelium of their free edges also fuses with the auricles beginning their final height at eye level and a 90 degree angle is formed between the leg and the foot the tail of the embryo has already disappeared and the external genitalia show some differences but they are so subtle that at this time it is still very difficult to ascertain the sex of the embryo, as you will see, the external changes that an embryo goes through during this period are multiple and diverse . I hope you have understood it and it will serve as complementary material in your study .

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