CLEAVAGE AND FORMATION OF THE BLASTODERM. 



Ill 



i;wo, four, and eight nuclei without separation of the individual blastomeres ; 

 the cleavage of the egg, which is at first complete, taking place only after these 

 preliminary divisions of the nucleus. From the sixteen-cell stage onwards the 

 egg then follows the superficial method of segmentation. 



Besides the forms already mentioned (Branchipus, Atyephyra, Eupagurus), 

 the following Crustacean eggs belong to this type of cleavage: — (1) The summer 

 eggs of many Cladocera {Polyphemus and Bythotrephes, according to Weismann 

 and Ischikawa, No. 6, the latter form having a blastocoele). (2) The eggs of 

 the Ostracoda (Cypris reptans, Weismann and Ischikawa, No. 6). (3) The eggs 

 of the free-living Copepoda (Claus, Nos. 18 and 19, Hoek, No. 22 ; Cetochilus, 

 Grobbex, No. 21 ; Cetochilus and Harpacticus, Van Beneden and Bessels, 

 No. 2). (4) Chondracanthus among the parasitic Copepoda (Van Beneden 

 and Bessels, No. 2), most of the Amphipoda (Uljanin, No. 75 ; Pereyas- 

 lawzewa and Rossijskaya, Nos. 70 to 73). From the observations of 

 La Valette St. George (No. 77), Van Beneden and Bessels (Nos. 1 and 2), 

 there appeared to be a considerable difference in the methods of cleavage of the 

 different species of Gammarus. G. locusta was said to belong to the type under 

 consideration, but the fresh- water species {G. pulex and fluviatilis) to the type 

 here ranked as third. Della Valle (No. 76), however, in confirmation of 

 the older observations of Leydig, has proved that in the latter cases also 

 cleavage is total in the first stages, so that we must class all the species of 

 Gammarus under 

 the present type. 

 (6) Some of the 

 Decapoda are, 

 perhaps, also to 

 be classed here ; 

 besides Eupa- 

 gurus and Atye- 

 phyra, Palaemon 

 (Bobretzky, No. 

 41) and Palaemo- 

 netes (W. Faxon, 

 No. 46) may pos- 

 sibly belong to 

 this type. 



This type of 

 cleavage also, 



perhaps, includes the Cirripedia, whose first stages of development seem to 

 follow a fairly simple course. In Balanus (Lang, No. 28 ; Hoek, No. 27 ; 

 Nassonow, Nos. 13 and 29 ; Nussbaum, Nos. 30 and 31) the cleavage appears 

 to be total, but somewhat unequal (Fig. 52), in which case we should have 

 an example of unequal cleavage in the Crustacea. The somewhat elongated egg 

 has one pole rounded and the other pointed. By the first cleavage, which takes 

 place horizontally or somewhat obliquely, the egg breaks up into two dissimilar 

 spheres ; the one near the rounded, i.e., the future anterior pole, consisting 

 entirely of formative yolk {a), yields the ectoderm, while that near the pointed 

 or posterior pole {b), which is rich in food-yolk, yields the elements of the 

 mesoderm and entoderm. The next division takes place in the ectoderm- 

 sphere and leads to the formation of a cap-like mass of cells (Fig. 52 B, C), 

 which gradually grow round the sphere containing food-yolk (Fig. 59 A, b). 



Fig. 52.— Three consecutive stages in the cleavage of Balanus (after 

 Lang). A, stage of division into two cells. B, the upper cell a has 

 divided into two. C, the same after division into four. 



