548 EMBKYOLOGY OF THE LOWEK VEKTEBKATES ch. 



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. ■ 



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 has not done so already, the Wolffian duct opens 

 into the cloaca. The mesonephric glomeruli begin to appear and the 

 tubules become elongated and coiled. In the posterior region of the 



Fio. 239. — Fowl's egg opened at the end 

 of the fifth day. The embr} r o enclosed 

 in its amnion is sunk down in the 

 centre of the vascular area, the allan- 

 tois projecting upwards towards the 

 serous membrane — a transparent mem- 

 brane through which the embryo and 

 allantois are seen. The increasing 

 fluidity of the yolk is shown by the 

 outward bulging of the yolk-sac wall 

 over the broken edge of the shell at the 

 lower side of the figure. The albumen 

 now lies completely underneath the yolk 

 so as to be invisible in >i view from 

 above. 



a.f, vascular area ; all, allantois ; *, bulging of 

 yolk over broken edge of shell. 



