CLEAVAGE OF ARTICULATES. 229 



in the very extensive articulated tribe (Arthropoda), in 

 Insects, Spiders, Centipedes, and Crabs. The Gastrula 

 which results from this form of cleavage is the Bladder- 

 gastrula (Peri-gastrula, Plate III. Fig. 29). 



In eggs which undergo this superficial cleavage, just as 

 in the eggs which have been mentioned, those of Birds, 

 Reptiles, Fishes, and other animals, the formative yelk is 

 quite distinct from the nutritive ; and the former is alone 

 concerned in the cleavage, which does not touch the latter. 

 But while in those eggs, the cleavage of which is discoidal, 

 the formative yelk is eccentric, and lies at one pole of the 

 single axis "of the egg, while the nutritive-yelk is massed 

 together at the other pole ; in those eggs, on the contrary, 

 which undergo a superficial cleavage, we find that the 

 formative yelk is spread over the whole surface of the egg, 

 surrounding the nutritive yelk in the form of a bladder, 

 which is central, and situated in the middle of the egg. 

 The cleavage, as it affects only the former, not the latter, 

 is naturally entirely superficial ; the provision, which is 

 massed in the centre, is entirely untouched by it. Other- 

 wise, this superficial cleavage proceeds quite regularly, like 

 the original cleavage, in geometrical progression (Plate 

 III. Fig. 25-30 represents several stages of this process in 

 perpendicular meridian section through the ellipsoid egg of 

 a Crab, Peneus.) The parent-cell, or cytula (Plate III. 

 Fig. 25), first parts into two similar cells ; from these, by 

 repeated simultaneous division, arise first 4 (Fig. 26), then 

 8, then 16 (Fig. 27), 64, 128, and so on. Finally, the whole 

 formative yelk parts into numerous, small, similar cells, 

 which lie side by side in a single layer over the whole 

 surface of the egg, forming a superficial germ- membrane 



