110 



CRUSTACEA. 



merited mass of yolk which fills the actual cleavage-cavity. Similar 

 yolk-pyramids were observed in Palaemon by BOBRETZKY ; here, 

 however, in the centre of the egg, they appear to be fused together. 

 This is also the case in Alpheus, Palaemonetes, and Hippo, (HERRICK). 



It has been observed that, in individual cases, not all the cleavage-nuclei shift 

 to the surface to form the blastoderm, but that some of them may remain in the 

 central mass of yolk (Atyephyra, ISCHIKAWA ; Crangon, KINGSLEY, No. 53). 

 The significance of these cells is not yet clearly understood. KINGSLEY believes 

 that they are belated cells, left behind in the process of blastoderm-formation. 

 It is, however, possible that they ought to be regarded as early representatives 

 of the vitellophags (cf. below, p. 134). 



A very primitive form of cleavage, to be ranked with the type above described, 

 is found in Branchipus, according to the unpublished researches of BRAUER 

 (Fig. 50). This form is distinguished by the fact that total cleavage is followed 

 for a long time, giving place only in the late stages to the superficial method,* 



and that a gradually increasing 

 blastocoele (/) appears early. 

 Such a blastocoele is not, as a 

 rule, to be observed in Crus- 

 tacean eggs, save in the type of 

 cleavage now under considera- 

 tion, the prismatic or pyramidal 

 blastomeres being usually in 

 contact at the centre. 



Careful examination of Crus- 

 tacean eggs that abound in yolk 

 and belong to this type reveals 

 the fact that, even in the first 

 stages, the blastomeres are 

 hardly able to contain the 

 mass of food -yolk supplied to 

 them, or to avoid fusing with 

 the neighbouring blastomeres. 

 ISCHIKAWA (No. 51) found in 

 Atyephyra, that, after division 

 into two the blastomeres once 

 more fused to form a single 

 ellipsoidal mass. In a similar 

 manner division into four is 



inaugurated by the separation of the four blastomeres, which, however, soon lose 

 their independence and completely fuse. Only in the later stages do the blasto- 

 meres become independent. Another instance of such disturbance seems to be 

 afforded by the peculiar type of cleavage observed by MAYER (No. 59) in 

 Eupagurus Prideauxii (Fig. 51). Here the first cleavage-nucleus divides into 



* According to Dr. BRAUER'S more recent researches, carried on since the 

 above was written, the process in Brancliipus seems to be somewhat different, 

 inasmuch as the last stage, which was regarded as showing superficial cleavage 

 (Fig. 50 D), has in reality passed into the stage of the formation of the germ- 

 layers, and the cleavage-cavity has become filled by the immigration of entoderm 

 cells. 



D 



FIG. 51. Four stages in the cleavage o.. Eupagurus 

 Prideauxii (after P. MAYER, from BALFODB'S Text- 

 book, bl, the completely formed blastoderm. 



