CLEAVAGE AND FORMATION OF GERM-LAYERS. 



39 



and equal cleavage. Each rosette, which has now become a simple 

 column, has a nucleus. In the further course of cleavage (Fig. 20 A) 

 the nuclei shift to the periphery, accompanied by the formative 

 protoplasm belonging to them. These, together with the periplasm 

 already present, separate from the yolk to form a peripheral layer, 

 which now contains the nuclei, and must thus be described as the 

 blastoderm (Fig. 20 B, bl). The yolk-columns, or rather pyramids, 

 may still be present at this time. Even earlier a cavity appears at 

 the centre, the cleavage-cavity (Fig. 20, B), the central yolk-mass 

 being withdrawn into the blastoineres as they develop, and pressing 

 further towards the periphery. 



The yolk-rosettes do not seem, as a rule, to be so distinct as 

 Ludwig found them in Philodromus. Yolk-pyramids have also been 

 seeninAgalena, 



Titer idium, ^ B 



Epeira, Phol- 

 cus, and other 

 forms, but the 

 groups formed 

 by them (the 

 rosettes of Phi- 

 lodromus) lie 

 closer to one 

 another (Fig. 

 21 ^4). A stage 

 in which there 

 are eight such 



groups closely resembles an egg that has undergone total and equal 

 cleavage, and that has a small cleavage-cavity (Fig. 21 B). Each 

 group of yolk-columns with its nucleus corresponds to a blastomere. 

 The blastoineres here also divide further, as in a case of equal 

 cleavage, and when, after repeated division, a large number of 

 blastomeres (about 128) have been formed, the nuclei, which have 

 meantime shifted to the periphery, with their protoplasm, separate 

 from the yolk below them, and thus give rise to the blastoderm 

 (Fig. 21 C and D). The cleavage -cavity, which may be fairly 

 large (Figs. 20 B and 21 G), becomes again filled with yolk, and 

 the regular arrangement of the latter is gradually lost (Fig. 21 

 D and E). The formation of the blastoderm seems to take place 

 more rapidly in the one half of the egg than in the other (Fig. 21 E), 

 (Salensky, Ludwig, Locy, Morin, Schimkewitsch). The former is 



Fig. 20.— Superficial aspect and optical section of a later stage in the 

 cleavage of Philodromus limoatus (after Ludwig, from Balfour's 

 Text-book). U, blastoderm ; yk, yolk-pyramids. In the space between 

 the vitelline membrane and the blastoderm the perivitelline fluid is 

 found (11). 



