DEVELOPMENT OF THE OVUM 8 1/ 



description is. illustrated in the text are those of Brucke, which were 

 photographed on wood from drawings made by Seboth. The Plates 

 in the atlas are photographic representations in color, of stained sections. 

 Figure 228 shows an early stage of development of the chick, in a 

 transverse section near the caudal extremity. In this figure, the upper 

 layer of dark cells (B, B) represents the epiblast. The lower layer of 

 dark cells (D, D) represents the hypoblast. The middle layer of lighter 

 cells is the mesoblast, which, toward the periphery, is split into two 



Fig. 228. Early stage of development of the chick about twenty-four hours (Seboth and Brucke) . 



layers. This figure represents a transverse section. At A, is a trans- 

 verse section of the groove that afterward is developed into the neural 

 canal. Beneath this groove is a section of a rounded cord (E\ the 

 notochord. The openings (G, G) represent the situation of the two 

 aortae. The other cavities are as yet indistinct in this figure (see 

 Plate XV, Fig. 3). 



Figure 229 shows the structures in a transverse section near the head. 

 The dorsal, or vertebral plates, which bound the furrow (A) in Fig. 

 228, are closed above, and include (A) the neural canal. The noto- 

 chord (E) is separated from the cells surrounding it in Fig. 228. The 



Fig. 229. Transverse section near the head (Seboth and Brucke). 



epiblast (B, B) and the hypoblast (D, D) present certain curves that 

 follow the arrangement of the cells of the mesoblast. By the sides 

 of the boundaries of the neural canal, are two distinct masses of cells 

 (C, C), which are the primitive somites. Outside of these masses of 

 cells, are two smaller collections of cells, afterward developed into the 

 Wolffian bodies. Beneath these two masses, are two large cavities 

 (G, G), the largest cavities shown in Fig. 229, presenting an irregular 

 form, which are sections of the two primitive aortae. The two openings 

 {//, H} afterward become the celom, or body-cavity (see Plate XV, 

 Figs. 4 and 5). 

 3G 



