CLEAVAGE AND DIFFERENTIATION 



reptiles and birds; superficial cleavage in eggs of some 

 coelenterates, of many insects and of some other arthropods. 

 Superficial cleavage might be said to be the mode among 

 arthropod eggs for with few exceptions in the eggs of the 

 members of this group that embraces the crabs, spiders, 

 insects etc., only the surface-located cytoplasm shows 

 cell-boundaries. 



In figures of the discoidal cleavage in the e^g of an ink-fish 

 or squid, Loligo, as seen in section, one notes at the upper 

 pole of the egg a heavily drawn line which represents the thin 

 sheet of superficial cytoplasm. It is in this sheet that the 

 cleavages ensue. In the unfertilized egg of a fish a thin 

 band of cytoplasm encloses the yolk. After fertilization 

 the superficial cytoplasm moves to the upper pole of the 

 egg to form a disc where cleavage now takes place. In 

 eggs of reptiles and of birds the process is the same: cleavage 

 Is limited to a small area of the total egg. As this disc 

 becomes cleaved, the underlying yolk is converted into 

 active cytoplasm which by cleaving adds to the area of 

 the original disc. 



Superficial cleavage was first clearly described by Weis- 

 mann in 1864. Since the appearance of this classic memoir 

 on the development of the insect egg, the clear superficial 

 cytoplasm of the egg, which encloses the egg-yolk and to 

 which during cleavage cell-boundaries are confined, has 

 been spoken of as the blastema. Following entrance of the 

 spermatozoon into the egg through a canal, the micropyle, 

 located at one pole of the ellipsoid egg, the egg- and sperm- 

 nuclei come together and form the cleavage nucleus located 

 below the egg surface. This nucleus divides mitotically sev- 

 eral times without cleavage of the egg. Later most of these 

 daughter nuclei become located in the blastema, a few 

 remaining in the central yolk-mass. With the arrival of 

 the nuclei in the blastema, cleavage planes appear in it, 

 the interior of the egg remaining uncleaved. 



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