LEPTOSTRACA, SCHIZOPODA, DECAPODA. 



155 



may here also be described _as optic lobes. The posterior pair of 

 formative centres (TA), which lie nearer one another, answer to the 

 unpaired caudal rudiment of Mysis. But, since here not only the 

 segments of the abdomen, but thoracic segments as well have begun 

 to form, these discs may be called the thoraco-abdominal rudiments. 

 The most posterior unpaired disc is the entoderm disc (ES). Anteriorly 

 to this there is a point where active proliferation yields cells which 

 spread out below the blastoderm-layer, this is the formative zone of 

 the mesoderm (BM). While, by processes already described in detail 

 (pp. 128, etc.), the invagination of the entoderm-disc and the gradual 

 closing of the blastopore take place, the thoraco-abdominal rudiments 

 come together to 

 form an unpaired 

 plate (Fig. 79, TA), 

 in the middle of 

 which the procto- 

 daeal invagination 

 (.4) is soon percep- 

 tible. The anterior 

 margin of this plate 

 then becomes more 

 sharply marked 

 by a transverse 

 depression (the 

 caudal fold), 

 which, in the fur- 

 ther course of de- 

 velopment, sinks 

 deeper, sloping 

 obliquely back- 

 ward. The tho- 

 raco-abdominal 



plate grows forward round and below this depression, forming 

 the long posterior portion of the body ; this is curved ventrally 

 and closely applied to the other embryonic rudiments (Figs. 80 and 

 63 B, p. 131). 



While these processes are going on in the posterior half of the 

 body, active growth takes place on each side in a band connecting 

 the optic lobes with the thoraco-abdominal rudiment (paired rudiment 

 of the germ-band), which finally lead to the development of three 

 pairs of limbs (Nauplius limbs, Fig. 79). These are the mandibles 



Fig. 78. — Part of the surface of an egg of Astacus ftuviatilis, with 

 embryo beginning to form (after Reichenbach, from Lang's 

 Text-book). BM, formative zone of the mesoderm; ES, ento- 

 derm-disc ; K, cephalic lobes (optic lobes) ; TA, thoraco- 

 abdominal rudiments. 



