. Text-book of embryology. Embryology. 548 EMBKYOLOGY OF THE LOWEK VEKTEBKATES The allantoic veins, which at first are merely veins of the body-wall, during the fourth day establish their connexion with the allantois, and in the course of the day the right vein disappears. â The allantois itself forms a conspicuous new feature for towards the end of the day it begins to project distinctly from the ventral side of the embryo about the level of the hind limb. Owing to the increasing size and complexity of the embryo the elementary student will not as a rule prepare complete series of sectio
. Text-book of embryology. Embryology. 548 EMBKYOLOGY OF THE LOWEK VEKTEBKATES The allantoic veins, which at first are merely veins of the body-wall, during the fourth day establish their connexion with the allantois, and in the course of the day the right vein disappears. â The allantois itself forms a conspicuous new feature for towards the end of the day it begins to project distinctly from the ventral side of the embryo about the level of the hind limb. Owing to the increasing size and complexity of the embryo the elementary student will not as a rule prepare complete series of sections later than the third day. He will however find it profitable to have transverse sections through the developing sense organs, sagittal sections through the head, and transverse sections through _ the posterior trunk region. From the study of sections the following advances in development during the fourth day may be made out. In the brain the rudiment of the paraphysis makes its appearance and the pineal outgrowth begins to sprout out into diverticula about the end of the day. The olfactory rudiment becomes connected with the buccal cavity by a slight groove. The rudiments of lagena and recess make their appear- ance as slight bulgings of the otocyst wall. The cavity of the lens becomes obliterated by the growth of its inner wall: pigment becomes conspicuous in the outer wall of the optic cup: the layer of nerve fibres in the retina becomes recognizable: mesenchyme begins to invade the cavity of the optic cup and about the end of the day also intrudes between the lens and the ectoderm. The post-anal gut becomes reduced to a solid strand of cells and finally disintegrates. The yolk-stalk becomes narrowed to a fine tubular channel. The gall-bladder begins to dilate towards the close of the day: the dorsal pancreas begins to develop outgrowths: and the rudiments of the caeca make their appearance. The mesoderm segments increase in number to about 50. Early in the day, if it ha
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