take part. Thus, the rudiments of the lungs appear in the 

 form of two hollow cone shapes. They develop by the same 

 method as the liver and the pancreas. The protrusions of 

 the caecum do not appear earlier than the end of the third 

 day; somewhat earlier, from the posterior end of the 

 digestive canal a single projection, the future urinary sac 

 (allantois), grows out. It is formed of two layers, an 

 external vascular and an internal mucous layer. 



In the second half of the third day, Baer detected the 

 appearance of "Wolff's bodies," as they were called. They 

 appear in the form of knotted thread-like cylinders in the 

 corner between the mesenteric and the abdominal layers 

 (Drawing 27, 6m). At the same time, on the abdominal layer, 

 the primary rudiments of the extremities appear with the 

 initial form of narrow cylinders. 



The spinal brain or cord during this period remains 

 compressed from the sides, but the walls become thicker and 

 subdivided into upper and lower layers. The divisions of the 

 cephalic brain are still poorly differentiated. Backwards 

 toward the marked hemispheres, the outlet of the optic nerves 

 appears. At the lower surface of the hemisphere arises the 

 basis of the olfactory nerve.. In the eye, one can distinguish 

 the retina and the lens. 



The separation of the embryo becomes more and more 

 distinct, because the ring-shaped interceptor between it and 

 the rest of the egg becomes narrow, forming an opening which 

 Baer called the umbilicus. A comparison of the successive 

 stages shows that the umbilicus was previously the wide 

 opening of the body. Even earlier, the umbilicus formed the 

 whole circumference of the opened body. The external layer 

 of the umbilicus is formed by the serous layer (Drawing 27, 7 

 P'> q f )» which from one side of the embryonic skin represents 

 a continuous extension and from the other side a surrounding 

 fold, i.e. the amnion. Inside the serous canal, which Baer 

 called "the skin umbilicus" or "abdominal umbilicus," there 

 is a tube composed of vascular and mucous layers, "the 

 intestinal umbilicus." The mucous and serous walls of this 

 tube form the passage of the vascular and mucous layers of 

 the blastoderm into the corresponding layer of the digestive 

 canal . 



324 



