ELEMENTARY VITAL PHENOMENA 



197 



themselves with a separate protoplasmic covering. Thus there 

 exists upon the whole surface an indifferent yolk-mass surrounded 

 by a single layer of separate cells (Figs. 77 and 78) — a phenomenon 

 that has been termed superficial cleavage. 



A special kind of multiple division is spore-formation, which it; 

 especially common in the Protista. The characteristic of this 



> ^^4 



%;,-•■ '^'^^^ 



Fig. 77. — Superficial cleavage of the egg of an insect in three successive stages. (After Bobretzky.) 



form of cell-multiplication is that the nucleus breaks up into a 

 very large number of tiny granules. Each of these small nuclei 

 surrounds itself with a certain quantity of protoplasm, so that tiny 

 cell-territories appear, which become free as amoebae or flagellated 

 cells, while the rest of the protoplasmic body perishes. The 

 swarm-spore, set free, represents a very 

 small cell containing a nucleus, and slowly 

 develops into the form of the protistan 

 cell from which it was derived. 



Finally, in reducing division, as Weis- 

 mann has termed certain processes that 

 lead to the formation of the ova and 

 the sperm-cells in the ovary and the 

 testis, a slight deviation in the behaviour 

 of the chromatic fibres of the nucleus 

 appears during division. The sperm-cells 

 arise by repeated division of other cells, 

 the sperm mother-cells. The first 

 division of the sperm mother-cells 



proceeds according to the type described above, but before 

 the nuclei have returned to the resting-stage a second division 

 takes places, each centrosome dividing into two halves which 

 diverge from one another and attract to themselves on both sides 

 the chromatic fibres that arise from the first division, without the 

 latter being able to split lengthwise as in the normal division. 

 Thus, one half of the chromatin-loops wander toward one pole, 



Fig. 7S. — Multiple division in 

 the cleavage of the egg of 

 an insect in two successive 

 stages. (After Balbiani.) 



