182 



INVEETEBEATA 



CHAP. 



cells ; but between the cephalic lobes there is still a groove of flattened 

 indifferent cells. As a result of these changes we have now a heart- 

 shaped, coherent, ventral plate of columnar cells. 

 ^_ In the next stage the blastopore changes from a circular to 



FIG. 130. Sagittal 

 section through a 

 portion of the em- 

 bryo of A stacus 

 ftuviatilis to show 

 the iuvaginatioii of 

 the endodermic 

 rudiment. (After 

 Reichenbach.) 

 end, endoderm, the two 

 references to eiul mark 

 the anterior and posterior 

 limits of the endodermic 

 plate; mes, "primary" 

 mesoderm ; mes 1 , second- 

 ary mesoderni. 

 % 



an elliptical shape, with its long axis coincident with the long 

 axis of the embryo. The thoracico-abdominal rudiments become 

 thoroughly united with one another in the middle line, and become 

 arched upwards so as to project over the open blastopore and partially 

 conceal it from view. Simultaneously the blastopore begins to close 



mes" 



end 



end 



th.abd 



FIG. 131.- Two sagittal sections through developing eggs of A stacus flitviatttw in order 

 to show the development of the endoderm. (After Reichenbach.) 



A, stage before the closure of the blastopore. B, stage in which the hind-gut has appeared. Up, 

 blastopore ; end, endodermic sac ; mes, mesoderm ; proct, proctodaeum opening by anus ; th.abtl, rudi- 

 ments of thorax and abdomen ; yi, endodermic cells swelling up to form secondary yolk pyramids. 



by the lateral union of its sides, the process beginning in front, and 

 its hinder border begins to grow forwards and thus assists in the 

 process of closing. 



Eeichenbach's account of this matter and his figures illustrating it 

 are most unsatisfactory. He denies that the backward growth of the 



