132 



CRUSTACEA. 



breaks up by an apparent secondary cleavage (the so-called cleavage 

 of the yolk) into spheres, each of which represents an entoderm- 

 cell ; these spheres are a homologue of the secondary yolk-pyramids 

 of Astacus. In later stages, the nuclei, each with a certain amount 

 of protoplasm, rise to the surface of these yolk-pyramids, and there 

 form an epithelium, which represents the wall of the mid-gut 

 (Fig. 64 (J), and which here, as in Astacus, contains within it the 

 food-yolk. Another group of the entoderm-cells, however, seems to 

 take no part in the formation of the mid-gut, but, remaining within 

 the yolk, shares its fate in becoming broken up and finally re-absorbed. 

 These cells must be considered as the homologue of the vitellophags 

 to be described later (p. 134) in connection with Mysis (cf. also 



BOBRETZKY, No. 80). 



ms 



4JI 



Fig. 64.— Three sections through the embryo of Palaemon, to illustrate the formation of the- 

 germinal layers (after Bobuetzky, copied from W. Faxon's Selections from Enibryologieal 

 Monographs). A, gastrala stage. B, closing of the mouth of the gastrula. C, longitudinal 

 section through a later stage, d, food-yolk; ec, ectoderm; en, entoderm; ep, midgut 

 epithelium ; g, ganglia of the ventral cord ; h, rudiment of the heart ; M, proctodaeal 

 invagication ; ms, mesoderm ; og, supra-oesophageal ganglion ; vd, stomodaeal invagination. 



In Homarus, according to Herrick (Nos. 50 and 50a), in place of the 

 invaginate gastrula, there is a quite shallow depression, starting from which a 

 solid wedge-like growth of cells (the keel) passes into the yolk. The cells 

 derived from this ingrowth soon absorb the yolk-elements. 



The immigration of amoeboid entoderm-cells into the food-yolk, and the 

 formation of the epithelium of the mid-gut at the surface of the latter appears, 

 in many cases, to be carried out in the manner already described. The relation 

 of the wandering entoderm-cells to the food-yolk varies, however, in individual 

 cases. Thus, according to P. Mayer (No. 59), in JSupagurus, after the immigra- 

 tion of the entodermal elements, the food-yolk undoubtedly breaks up into a 

 number of Irregular portions and undergoes a kind of re-arrangement, although 





