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 (7), 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 



X 



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

 germinal layers (after BOBRETZKY, copied from W. FAXON'S Selections from Embryological 

 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, mid -gut 

 epithelium ; g, ganglia of the ventral cord ; Ji, rudiment of the heart ; lid, proctodaeal 

 invagitation; 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 Eupagurus, 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 



