DEVELOPMENT PRIOR TO LAYING 



55 



The margin of invagination is known as the Up of the blasto- 

 pore or primitive mouth; the space between this margin and 

 the yolk is the blastopore, and the space between the entoderm 

 and yolk, derived from part of the subgerminal cavity, is the 

 archenteron or primitive intestine. 



Fig. 27. — Diagrammatic reconstruction of the blasto- 

 derm of which a longitudinal section is shown in 

 Fig. 26. 



C-D., Plane of Fig. 26. 



G. W., Germ-wall. 1, 2, 3, 4, 5, 6, and 7 indicate 

 regions of the blastoderm which are approximately from 

 1 to 7 cells deep respectively. The broken line around 

 1 indicates the region where the blastoderm is approxi- 

 mately one cell deep, x 27.2. (After Patterson.) 



The first stage in the formation of the entoderm is interpreted 

 as involution of the free margin, and this view is supported by 

 the fact, determined by Patterson, that the antero-posterior 

 diameter of the blastoderm is shorter than the transverse diameter 

 during this process, whereas previously the blastoderm was 

 approximately circular. An even stronger support of this view 

 is furnished by experiments which demonstrate that injuries to 

 the margin made just prior to gastrulation appear later in an 



