CLEAVAGE AND FORMATION OF THE BLASTODERM. 109 



of yolk within it. First two and then four portions become separated 

 from this cell (Fig. 49 C, d), and these, shifting out of connection 

 with the entoderm, come to lie within the primary body-cavity at 

 the apex of the archenteric invagination. The significance of these 

 portions of this cell is still doubtful (cf. below, p. 127). 



Type II. Eggs with total cleavage in the first and superficial 

 cleavage in the later stages. This type is very common among the 

 Crustacea. The cleavage here begins with a total and, in most cases, 

 an equal division (cf. Fig. 50 B and C), the egg dividing up first into 

 two, then into four, eight, and sixteen cleavage-spheres of equal size, 

 which are similarly filled with spherules of yolk. Within each of 

 these cleavage-spheres lies a nucleus surrounded by a star-like mass 

 of protoplasm which sends out numerous processes. The further 

 cleavage proceeds, the more do these separate nuclei approach the 

 surface of the egg. As a consequence of this they lose control of 

 those portions of the now prismatic cleavage-cells which stretch 

 inwards. The result is a stage in which cell-regions can be dis- 

 tinguished at the surface divided by furrows, while in the interior 

 of the egg the originally distinct cells have become secondarily fused 

 together (Fig. 50 D). The cleavage has become superficial. At the 

 same time an even sharper distinction between formative and nutritive 

 yolk takes place. The superficial cells finally contain only formative 

 yolk, and become separated from the nutritive yolk by a distinct line. 

 A blastula-stage (Fig. 50 D) is thus finally reached ; this consists of 

 a superficial layer of cells of equal size, and of an inner mass of 

 yolk (now apparently filling the cleavage-cavity*). In the latter, no 

 distinct demarcation between the portions belonging to the separate 

 blastoderm-cells can, as a rule, be made out. There are, however, 

 indications of such demarcation in the form of radial furrows, which 

 are to be seen specially distinctly in the egg of Astacus (Fig. 55, 

 p. 114) belonging to the next type of cleavage. In this egg, the 

 central mass of yolk breaks up into the so-called primary or 

 Rathke's yolk-pyramids (observed later by Lereboulet, No. 58, and 

 Bobrbtzky, No. 41) and a spherical central body (Reichenbach, 

 64, 45). The yolk-pyramids here represent the yolk of the 

 separate blastomeres, while the central mass represents the unseg- 



* Strictly speaking, the food-yolk does not lie in the cleavage-cavity, but 

 oocupies a considerably larger space than did the original cavity. We must 

 therefore distinguish two parts in the food-yolk : a central portion which fills 

 the original cleavage-cavity, and a peripheral portion corresponding to the fused 

 inner ends of the blastomeres. Only the distal portions of the blastomeres have 

 entered into the formation of the blastoderm. 



