GENERATION OF INSECTS. 407 



condition of a minute pellucid vesicle, having a central nucleus. 

 Such nuclei and nucleated cells make their appearance in the capil- 

 lary beginnings of the ovarian tubes, where they are drawn out to 

 microscopic tenuity. From these extremities the ova successively 

 pass into the wider part of the tubes, and in this course increase in 

 size by the expansion of the nucleus, and by the multiplication of 

 vitelline granules around the primitive cell ; at first the ova are sepa- 

 rated from each other by an amorphous granular substance of equal 

 size, which is called a placentula, but lower down by mere constric- 

 tions of the ovarian tube. Here the ova acquire a distinct vitelline 

 membrane, and then, continuing to increase in bulk by the addition 

 of vitelline matter, they reach the converging end of the ovarian 

 tubes, and enter the shorter and wider oviduct. In this tube they 

 receive additions to their external surface from the secretions of the 

 colleterial organs, and admit into their interior the mysterious 

 principle of the male fluid, which would seem to be assimilated into 

 their substance, more especially into that of the central nucleated 

 germ-cell. 



The eggs of the Lepidoptera, Diptera, Hymenoptera, and some 

 Coleoptera {CicendelidcB, Creabidce, e. g.) are formed on a different 

 type. The rudimental ova, or vitelline masses, are separated by 

 groups of large vitelline cells, whose contents are blended with the 

 subjacent ovum. The chorion is formed by a layer of vitelline cells, 

 and gradually extends over the vitelline mass, closing at the upper 

 portion of that mass. The eggs have reached maturity at the end of 

 the pupa state, in the Lepidoptera, so that they are ready for im- 

 pregnation and oviposition as soon as those insects have cast off the 

 pupal envelope. But in the Agrion and Libellula they are not 

 matured until a later period. 



It is essential to the development of the embryo, that the germ-cell 

 receive the matter of the spermatozoon ; the ovum is then said to be 

 impregnated. The phenomena that thence ensue are essentially the 

 same up to a certain point in all animals, and consist in the propaga- 

 tion on the part of the impregnated germ-cell, by a series of reite- 

 rated spontaneous divisions, of a numerous offspring. A right com- 

 prehension of the purpose of this process, or the object effected by 

 it, is essential to the elucidation of the nature and relations of the 

 subsequent modifications and varieties in the course of development. 

 The progeny of the primary impregnated germ-cell are the "secondary 

 or derivative germ-cells," and the whole is the " germ-mass." 



This progeny resembles the parent-cell in all respects, save that they 

 show a diminution of size. When they cease to exist as germ-cells, 



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