400 



COMPARATIVE ANATOMY 



CHAP. 



Fio. 2V0.— Astacus fluTlatilis, section througli an egg, after 

 completed formation of blastoderm. c7;, Choriuii ; k^, stalk of 

 attachment ; M, blastoderm cells. The nutriti\'i^ yulk is black in 

 this and the following Fig.s. (after Reichenbach). 



Fig. 271.— Astacus fluviatilis, part of the surface of an egg 

 witli embryo beginning to form.— Stage A. K Cephalic lobes, 

 with the rudiment of the eye ; TA, thoraco-abdominal rudiments ; 

 BM, formative zone of the mesoderm ; ES, endodenn disc (after 

 Eeichenbaoli). 



Stage B. Embryo, with 

 semicircular gastral fur- 

 row. — Only the ventral 

 plate alters. At the an- 

 terior edge of the endodenn 

 disc a semicircular furrow, 

 or a fold projecting inwards, 

 ai^pears. 



Stage C. Embryo, with 

 circular gastral furrow. — 

 The thoraoo - abdominal 

 plates unite in the middle 

 line. The semicircular 

 furrow has become a fur- 

 row round the whole cir- 

 cumference of the endoderm 

 disc and is thus circular. 

 The middle part of the en- 

 doderm disc sinks down, so 

 that it now becomes a de- 

 pression with a somewhat 

 raised floor (gastrula inva- 

 gination). This depression 

 represents the rudimentary 

 midgut, and the outer edges 

 of the original circular 

 furrow are the edges of 

 the blastopore or gastrula 

 mouth. In front of the 

 anterior edge of the blasto- 

 pore lie the primitive meso- 

 derm cells in active growth, 

 and giving off cells towards 

 the centre of the blasto- 

 sphere (Fig. 276). 



Stage D. The primitive 

 mouth is in the act of 

 closing. — In the centres of 

 the two cephalic lobes are 

 the rudiments of the eyes. 

 Between the cephalic lobes 

 and the thoraco-abdominal 

 rudiments the first traces 

 of the mandibles and an- 

 tennae appear. The primi- 

 tive mouth closes from be- 

 fore backward, its lateral 

 edges growing together in 

 the middle line. The cells 

 of the primitive intestine 

 begin to consume nutritive 

 yolk. 



