116 CRUSTACEA. 



already present in the blastoderm-disc. It is probable that these two types have 

 been confused one with the other, since in many cases in which the occurrence 

 of discoidal cleavage in Crustacea has been maintained, the observations \vere 

 not confirmed by the systematic examination of sections. It therefore appears 

 probable to us that most parasitic Copepoda, to which VAN BENEDEN and 

 BESSELS (No. 2) ascribed discoidal cleavage, in reality develop according to- 

 Type III. b. The same is possibly the case with the Isopoda (Oniscus, Ligia*}, 

 for which discoidal segmentation was asserted by BOBRETZKY (No. 80) and VAN 

 BENEDEN (No. 1). We are, indeed, justified in raising the question whether 

 true discoidal cleavage ever occurs in the Crustacea, and whether more careful 

 examination might not lead to the complete absorption of Type IV. in 

 Type III. b. Among the Decapoda also there are some representatives of this 

 last type, such as ffomarus, Eriphia, and perhaps also Palaemon. It is possible, 

 however, that the last-named genus, on account of the total (?) cleavage that 

 takes place in the first stages, should, like Atyephyra, be assigned to Type II. b. 



Type IV. Eggs with discoidal cleavage. In the types of 

 cleavage which we have so far considered, two processes go on 

 side by side simultaneously, viz., (1) the increase of the elements, 

 and (2) the separation of the blastomeres from the food-yolk (i.e., 

 the separation of the plastic portion of the egg from the nutritive). 

 In the blastula stage, which finally results in Types II. and III., we 

 then find a superficial layer of epithelium and an inner mass of 

 food-yolk, in which, as a rule, no cell-nuclei or other plastic elements 

 are still to be found. If we now imagine this process of separation 

 between the blastomeres and the food-yolk to be shifted back to the 

 earliest stages, we obtain an explanation of discoidal cleavage, as it 

 has been observed in Mysis (VAN BENEDEN, 'No. 37 ; NUSBAUM, 

 Nos. 38 and 39) and Cuma (No. 35), as well as in some other 

 forms, f Here the very first cleavage-cell becomes entirely separated 

 from the food-yolk, on the surface of which it comes to lie. The 

 food-yolk from this time onward contains no more cleavage-nuclei. 

 The formation of the blastoderm begins with the superficially placed 

 cleavage-nucleus, which divides (Fig. 56 A), and thus yields a cap of 

 blastomeres (Fig. 56 B) ; these increase in number by continual 

 division, and gradually grow over the whole surface of the sphere 

 of food-yolk. The starting-point in the formation of the blastoderm 

 here corresponds to the spot where, later, gastrulation takes place 

 (posterior end of the ventral side of the embryo), while the blasto- 

 derm is finally completed on the dorsal side. At this ventral 

 starting-point of the blastoderm there is from the first a thicker 



* Quite recently NUSBAUM has again maintained the presence of discoidal 

 cleavage in Ligia (No. 85a). 



f [BouTCHiNSKY (Zool. Anz., xx., 1897, p. 219) describes a discoidal seg- 

 mentation in Nebalia. ED.] 



