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 



Fio. 130. — Sagittal 

 section through a 

 portion of the em- 

 bryo of Astacus 

 iluviatilis to show 

 the invagination of 

 the endodermie 

 rudiment. (After 

 Reichenbach.) 

 eiidf endoderra, the two 

 'Cferences to end mark 

 t,he anterior and posterior 

 limits of the endodermie 

 jilate ; mes, "primary" 

 mesoderm ; mesi, second- 

 ary mesoderm. 



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 



Fig. 131. — Two sagittal sections through developing eggs o( A stacus flwviatilis in order 

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



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

 blastopore ; end, endodermie sac ; ines, mesoderm ; proct, proctodaeum opening by anus ; tli.ahd, rudi- 

 ments of thorax and abdomen ; yp^, endodermie 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 



