70 



eight oviducts, Individual development is checked and 

 arrested at the apterous larval condition. It is plain, there- 

 fore, that the essential condition of the development of 

 another embryo in this larva is the retention of part of 

 the progeny of the primary impregnated germ-cell. 



What is really surprising in the phaenomena of the 

 Aphides is the potency of the mysterious virtue of the 

 quintessential excretion, which sustaining so great a de- 

 gree of subdivision, and of dilution with the material incor- 

 porated in the successive generation of cells, is nevertheless 

 equal to the renewal and repetition of embryonic develop- 

 ment through so many generations. 



The explanation of the condition essential to this pro- 

 cess is still more clearly afforded by the development of the 

 Distoma tarda as described by Steenstrup. The entire 

 impregnated ovum consists, as in most of the lower Inver- 

 tebrata, of the germ-cells, which have assimilated the whole 

 of the germ-yelk; the ovum, therefore, grows with the 

 growth of the germ-mass which is formed by the usual pro- 

 cesses of imbibition and spontaneous fission producing the 

 multiplication of cells and nuclei : but certain of the outer- 

 most of these nuclei liquefy or coalesce, and form an addi- 

 tional layer to the chorion of a contractile or a ciliated 

 structure. The ovum is thus transformed into a sub- 

 elongate, subdepressed locomotive animal ; which contains, 

 however, nothing in its interior except the multiplied off- 

 spring of the primitive germ-cell. Here, therefore, we have 

 the condition which I regard as essential to the development 

 of an animal under circumstances which Steenstrup would 

 define as ( alternate generation by the vital powers and by 

 means of the bodies of the producing individuals/ and 

 wiiich the reviewer who has criticised him would prefer 

 to call a development ' by a multiplication of cells by a 

 process of continuous growth/ Neither of these definitions 

 explain the condition to which I refer. That condition is 

 the existence in each of the cell-progeny of the primary 



