it represents a division from both other layers, and the 

 animal part of the embryo develops from it. 



This division of the animal and plastic layers Baer 

 distinctly implemented in his first volume (§ 1) and in the 

 fourth scholium of the second part of UBER ENTWICKLUNGS- 

 GESCHICHTE. Here he noted the importance of such opposition 

 for the comparison of embryonic development of different 

 vertebrates and for the comparison of development of the 

 vertebrates and the lower animals. "The vegetative layer," 

 Baer wrote, "contains the layer of the mucous membrane and 

 the vascular layer, but the animal layer at first corresponds 

 with Pander's serous layer. Later it divides in the middle 

 into two layers, the lower of which I call the fleshy layer 

 and the upper the skin layer" (II, 51, pp. 46 - 47 fn. (64)). 



The embryo, on separating from the yolk, connects with 

 the remaining parts of the egg by means of the umbilicus. 

 The external umbilicus represents the border between the 

 embryonic and extra- embryonic parts, which previously repre- 

 sented the periphery of the animal layer. The internal 

 umbilicus, the yolk duct, represents the transition of the 

 internal vegetative parts into the vegetative layer of the 

 embryonic sac. In the vegetative layer of the yolk sac there 

 are two subordinated layers, a vascular and a mucous layer. 

 The blood- carrying vessels, present only in the former, are 

 transferred into the embryonic vessels. In mammals they are 

 called the umbilical-mesenteric vessels because they go 

 through the umbilicus from the mesentery. The animal layer 

 of the rudiment undergoes transformation upon the appearance 

 of the amnion on the third or fourth day, when the blastoderm 

 is divided into two main layers. 



Baer's description of amnion development is very unclear. 

 He proposed to withdraw Wolff's idea of the "false amnion" 

 (see Chapter 5) from use, especially since Pander did not 

 employ it in the same sense (see Chapter 12) ; this produced 

 some confusion. 



Baer designated the cephalic fold of the animal layer 

 the cephalic cap; it gradually enlarges and forms the cephalic 

 vagina. Slightly later, this same process takes place at the 

 caudal or tail end (the caudal cap becomes the caudal vagina) 



378 



