CLEAVAGE AND FORMATION OF THE BLASTODERM. 



113 



lias been observed by Kossmanx, seems here also to be a secondary process 

 occurring in the later stages. As the cleavage of Saccxdina aj^pears to belong to 

 the Type II. b, described below, p. 115, it tends to confirm the view of the 

 cleavage of Balamis to which preference was given above. 



Type III. Eggs with purely superficial cleavage. In this 

 type, the formative yolk, from the very beginning, 1ms no control 

 over the mass of food-yolk. The first cleavage -nucleus, lying in 

 the centre of the egg (Fig. 53 A), divides regularly into two, four, 

 eight, etc., nuclei (Figs. 53 B-D and bi A), which are surrounded 

 by radiate masses of protoplasm. The areas of the separate cells, 

 however, do not become marked off by furrows cutting right through 

 the egg, although, in a few cases, such furrows are indicated even 

 in early stages as grooves on the surface (Fig. 53 E). The larger 

 the number of cleavage -nuclei becomes, the more do they shift 

 towards the surface (Figs. 53 D and 54 B), and a regular blastoderm 

 is formed in the same way as that described under Type I. (Fig. 

 53 F-H). 



Fig. 54. — Two stages in the cleavage of the egg of Astacus (after Morin). A, younger stage 

 with a few cleavage-nuclei within the egg. B, older stage with superlicial distribution of 

 the cleavage-nuclei and a correspondingly wavy surface. 



The processes here described as taking place within the egg, i.e., the division 

 of the cleavage-nnclei and the shifting apart of the radiate islands of protoplasm 

 which surround them have often been called cleavage. Indeed, these islands 

 of protoplasm have even been named cleavage-cells, which then appear in a 

 certain contradistinction to the masses of food-yolk. In so far, however, as 

 we regard the wJiole egg as the equivalent of a cell, and the two, four, eight, etc , 

 .separate spheres produced by its total cleavage as cells, it is evident that we 

 ought not to consider these protoplasmic islands, which arc designated " cleavage- 

 cells," as fully equivalent to blastomeres. They represent merely the centres 

 of blastomeres, whose areas, owing to the absence of furrows, have not been 

 demarcated. In tlie first stages of superficial cleavage, the egg is on the level 

 of a multinucleate cell. Tiiis view is supported by the frequently observed fact 

 tliat the so-called "cleavage-cells" are connected together by a reticulum of 



