228 P. G. HEATHCOTB, 



■which the embryo is finally hatched. The somites are at first 

 solid, but a cavity appears in them at a later period. 



The ectoderm of the ventral plate now alters its character, the 

 cells becoming more pointed and much more closely packed 

 together. 



From the Formation of the Stomodseum and Proc- 

 todaeura to the Hatching of the Embryo. 



Early on the ninth day the stomodaeum is formed as an in- 

 vagination of the ectoderm near one end of the ventral surface. 

 Shortly after the first formation of the stomodseum the proc- 

 todeeum appears as a shallow^ somewhat wide invagination near 

 the end of the ventral surface. 



The body segments, already established by the segmentation 

 of the mesoderm, now become more apparent, each being 

 marked by a deep transverse furrow in the ectoderm (figs. 24, 

 25, 28). Fig. 12 is a section taken longitudinally through the 

 embryo, and shews the storaodfeura, the proctodseum, the eight 

 mesodermal segments, and a single ectodermal furrow close 

 behind the stomodaeum. Fig. 24 shows this first furrow under 

 a higher power. (Zeiss c.) 



The endoderm cells are still scattered within the yolk, but 

 they are gradually becoming collected in the median line just 

 below the mesoderm. The stomodseum and proctodseum 

 become more deeply invaginated, extending a considerable 

 distance into the yolk and at the same time the endoderm cells 

 begin to form the mesenteron, arranging themselves round a 

 central lumen. Fig. 27 shows the formation of the proc- 

 todeeum and the hypoblast cells beginning to form the 

 mesenteron. 



At the end of the ninth day, then, the embryo is of a long 

 oval shape, with a deeply invaginated stomodseum at the 

 anterior end and a proctodseum not quite so deep at the other; 

 the mesoderm is divided into eight segments ; a deep furrow in 

 the ectoderm marks off the first segment which will eventually 

 become the head, and the mesenteron is almost formed. 



