THE BIOLOGY OF THE CELL SURFACE 



during cleavage; the mass of protoplasm does not increase 

 but the total surface-area does. Increased surface-area 

 is best seen in an egg with total cleavage which develops 

 into a blastula. It is clearly shown by the egg of the star- 

 fish for during early cleavage the blastomeres are at first 

 separated from each other. In totally cleaving eggs which 

 give rise to morulae, the increased surface area appears in 

 the covering blastomeres; they are more flattened and the 

 ratio of surface to mass thus is greater. For eggs with 

 discoidal cleavage, it has been established that new cells 

 are added to the embryonic disc from the underlying yolk 

 where nuclei undergo mitotic division without being par- 

 titioned off into cells; only when around those nuclei lying 

 next to the disc cell-boundaries form, are new cells added 

 to the growing embryonic disc. Cleavage-planes arise in 

 superficially cleaving eggs only after the nuclei reach the 

 ectoplasm. The difference between totally and partially 

 cleaving eggs lies in the fact that the initial cleavages in 

 the latter are confined to an ectoplasmic region which is 

 utilized in the formation of cell-walls before additional 

 ectoplasm forms; in the former, cleavage and increase in 

 ectoplasm run more synchronously. 



Since the ectoplasm is a part of the living system and 

 since during cleavage no living substance is added to the 

 egg from the outside world, the source of new ectoplasm is 

 from the egg-substance itself: ground-substance alone 

 constitutes this source. 



But the interpretation of the role of the ectoplasm in 

 differentiation does not rest only upon its amount and its 

 source; in some of the events named as occurring during 

 the cleavage-period, the ectoplasm plays a part. 



In the first place, consider the fact that a fertilized egg, 

 which possesses pluripotency, develops normally only into 

 one embryo. This means that only if the ectoplasm of the 

 blastomeres is removed from its normal contact with the 



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