114 



CRUSTACEA. 



fine protoplasmic processes. If, tlierefore, we understand by cleavage here, 

 as elsewhere, the act of marking off separate cell areas, it follows that the term 

 superficial cleavage is applicable to the present type, since cleavage actually 

 occurs here only in the superficial parts of the egg. 



This type of segmentation is also very common among the Crustacea. It 

 is found in the following eggs: — (1) In the summer eggs of many Cladocera 

 {Moina, Daphnia, Sida, Leptodora, DaphncUa, Weismanx and Ischikawa, 

 No. 6), and in all ivinter eggs (Moina, Daphnia, Sida, Bgthobxplies, Pohiphemus, 

 Leptodora, Weismaxn and Ischikawa, No. 16). There is thus among the 

 Cladocera a group of forms {Bi/thotrejjhes, Polyphemus), the summer eggs of 

 which belong in their cleavage to Type II., while the winter eggs belong to 

 Type III. (2) In the eggs of several Isopoda [Asellus,* Van Beneden, No. 79; 

 Porccllio, Reinhaud (No. 91) and Roule (No. 92). This type of cleavage 

 may, perhaps, be more common among the Isopoda than has hitherto been 

 thought. (3) In the eggs of Penaeus (Haeckel, No. 47), Callianassa sub- 

 terranea (Mekeschkowski, No. 60), Astacus (MoiiiN, No. 61), Homarus 

 (Hekrick, No. 50a). 



B 



Fio. 55.— Later stages in the cleavage of the egg of AstacMs (after Reichenbach, from 

 Hatschek's Text-hook). A, section of one of the stages. The formative yolk has collected 

 at the surface. The food-yolk has divided into separate yolk-pyramids. The central body 

 is found within it. B, later stage in which the layer of blastoderm-cells {1) has become 

 distinct from the yolk-pyramids (2). 



It is very important, in the formation of tlie blastoderm Avhich 

 follows the superficial cleavage, to distinguish clearly two subsidiary 

 methods : 



(a) Tlie formation of the Uastoderm talces place all over the surface 

 simultaneously, e.g., in Astacus, Brancliipus, and the free-living 

 Copepoda. 



* According to more recent statements by Roule (No. 92), which are not 

 very clear, it appears as if the cleavage of Asellus were at first total and only 

 later superficial. Van Beneden, on the contrary (No. 79), emphasises the fact 

 that at first a mere increase in the number of nuclei within the yolk takes place, 

 that these nuclei become distributed later at the surface of the egg, and that it is 

 at the surface that the limitation of their areas occurs, while nearer the centre 

 the mass of yolk remains unfurrowed. In this case, the egg of Asellus undoubt- 

 edly belongs to Type III. 



