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TEXT-BOOK OF ENTOMOLOGY 



conspicuous rays, like those observed in ordinary cell-division. Pre- 

 ceded by these rays, the sperm-nucleus or male pronucleus (Fig. 502, p~) 

 moves towards the nucleus of the egg, and finally fuses with it, thus 

 forming a new single nucleus. This latter, which is called " the cleav- 

 age nucleus," rapidly forms a nuclear or " cleavage spindle " (Fig. 

 503). This act gives an impulse to the cleavage of the egg, which is 

 the first step in the formation of the embryo. All these changes 

 have yet to be worked out in detail in insects by microscopic sec- 

 tions of the egg, whose generally hard and opaque egg-shells present 

 great obstacles to such work. 



(7. Division and formation of the blastoderm 1 



In insects as in most other Arthropoda the segmentation of the 

 yolk is superficial and not total. The ovum is centrolicithal, i.e. the 

 yolk is concentrated at the centre of the egg, and surrounded by a 

 peripheral layer of transparent protoplasm (the Keimhautblastem). 



The first step in segmentation is the movement of the first division- 

 nucleus (i.e. that in the fertilized egg arising from the union of the 



C 



Fio. 504. Formation of the blastoderm of Pieris c.ratwgi : A, longitudinal section through 

 the ri_'L r . with twn m:i>>cs of protoplasm in the yolk. />', a blastoderm-cell at the upper end. ('. :i 

 later .-UiL r '-, with more blastoderm-cells. After Bobi'fetsky. 



sperm-nucleus with the female pronucleus) towards the interior of 

 the egg in order to multiply itself by the mode of indirect nuclear 

 division (Figs. 504, A, and 507). 



i In the following general account of the embryology of insects, I have closely 

 followed the admirable arrangement and description of Korschelt and Heider, in their 

 Lehrbuch der vergleirhenden Entwicklungsgeschichte der wirbellosen Thiere, pp. 764- 

 840, often translating their text literally, though not omitting to state the results of 

 other writers. 



