and divided into three cavities. The head portion now 

 reaches the place of bifurcation of the heart; here also is 

 formed "the heart depression," leading to the esophagus. 

 The lateral borders of the head portion are strongly stretched 

 towards the tail. At the posterior (the lower, according to 

 Pander) end of the fetus appears the wrapped-up part of the 

 blastoderm, which later forms the tail portion, whose 

 borders go along the sides of the embryo and get into the 

 folds near the heart depression. The blood islets at this 

 stage acquire a red color. 



By the end of the second day (§ 10) the depression, in 

 which the free anterior part of the embryonic body is 

 situated, increases; the head portion, which is outlined 

 by a half-moon border, is distinctly formed. Then the fetus 

 turns to the left side. The granular layer is converted 

 into the vascular plate, which is situated between the two 

 layers of the blastoderm; the latter, therefore, becomes 

 three- layered. The blood islets of the middle layer have 

 now the shape of vessels with their own walls, i.e. they 

 are converted into blood vessels. The spaces, formed by 

 the bent primary folds in the head region become filled with 

 vacuoles and are converted into cavities from which the brain 

 is formed. The posterior (first) space gives the origin to 

 the medulla oblongata, the second to the rounded body of the 

 four hillocks, and the third the narrower part of the brain, 

 while the stems and protuberances of the optic nerve and the 

 most anterior (fourth) is converted into the hemispheres. 

 Pander gave special attention to the fate of the lower 

 space which is adjacent to the yolk surface of the blasto- 

 derm. He described two pairs of folds there, the external 

 and internal. The external are formed from all the layers 

 of the blastoderm; and the internal only from the vascular 

 and mucous layers. The external folds, in Pander's opinion, 

 form the peritoneal walls and share in the formation of the 

 intestines. He calls them the peritoneal and intestinal 

 folds. To the internal folds, in correspondence with their 

 subsequent fate, he gave the name of the mesenteric folds. 

 Undoubtedly, these paired folds correspond with those of the 

 embryonic formations which Baer later called the peritoneal 

 and intestinal layers. In this place Pander made a general 

 footnote of several pages devoted to Wolff's investigations 

 and the evolution of Oken's work. 



254 



