Development of the Fourth Day 



22 



until now the splanchnic stalk is reduced to 

 a very narrow but distinct tube, connecting 

 the yolk-sac with the nearly enclosed mid-gut. 

 The somatic stalk has not progressed so far, 

 so that there is a ring-shaped space between it 

 and the splanchnic stalk : through this space 

 projects the allantois, which is very much in 

 evidence as a large, sac-like object, connected 

 by a narrow stalk with the hind-gut, just in 

 front of the well-developed tail (Fig. 72, all). 



The cranial flexure increases to such an ex- 

 tent, during this day, that the front end of the 

 head forms an acute angle with the neck and 

 hind-brain ; and by the increase of the body 

 flexure as well, the outer curvature of the em- 

 bryo forms almost a semicircle, and the fore- 

 brain and tail are only a short distance apart 

 (Fig. 72). 



The embryo continues to increase in depth 

 (compare Figs. 71 and 73), and the muscle 

 plates are now nearly vertical in position, and 

 extend to about the point of separation of the 

 somatopleure and the splanchnopleure. Just 

 beyond this point of separation, the somato- 

 pleure is elevated to form a longitudinal ridge, 

 on each side, known as the Wolffian ridge 

 (Fig. 71). 



