DEVELOPMENT PRIOR TO LAYING 



55 



The margin of invasination is known as the hp of the blasto- ] 

 pore or primitive mouth; the space between this margin and 

 the volk is the blastopore, and the space between the entodem -^ 

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

 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. ■ - ■ i- , 



G. W., Germ-wall 1, 2, 3, 4, 5, 6, and ' >™lioate 

 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- 

 mLtely one cell d?ep. 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-poster.or 

 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 bv experiments which demonstrate that injuries to 

 the margin made just prior to gastrulation appear later in an 



