vascular area outside the embryo into the embryo between the 

 serous and mucous layers, indistinctly delineated from both. 

 "All the rudiment," Baer wrote, "can be considered as the 

 unformulated body of the whole animal, which appears as 

 nothing other than a large unclosed intestinal sac" (I It, 

 p. 51 (20)). 



The spinal plates Baer considered the expansion of the 

 serous layer; in a footnote (I It (20), he discussed Pander's 

 terms "serous layer" and "vascular layer" and approached a 

 modern subdivision of the embryonic layers. Baer remarked 

 that Pander's experiment "distinguishing the layers of the 

 rudiment membrane was a turning point in the study of the 

 history of development and was a true light for the latest 

 investigation." Baer, however, considered that the early 

 separation of the layers only serves as a preparation for 

 future development. 



Running somewhat ahead, he stated that at the end of 

 the second day he could already clearly distinguish in both 

 the embryo and the rudiment membrane the animal and its 

 plastic or vegetative parts. The first consists of two 

 layers, the future skin and the animal part of the body, 

 and the second is composed of the vascular and mucous 

 layers. And thus, the most characteristic picture for the 

 first day of incubation, besides the separation of the layers, 

 is the "growth of the embryo from the primordium which is 

 yet on the anterior end, as a result of which the primordium 

 is divided into the embryo and the primordial membrane" 

 (I lu, p. 52 (21)). 



The extensive second section Baer devoted to the events 

 of the second day of incubation which completes the first 

 period of development. During the second day, the separation 

 of the embryo from the yolk continues, which leads to the 

 lifting of the anterior half of the body. 



The previously described accretion of the spinal plates 

 does not occur at the same time along the entire length; 

 rather, it starts in the region behind the head and continues 

 to the region of the future sacrum. The number of primary 



315 



