74 



of the germ-mass and embryo in ovo : it is the primary im- 

 pregnated germ-cell. 



A derivative germ-cell, the progeny of the primary one, 

 retaining a due amount of spermatic force, is essential to 

 the development of a secondary germ-mass and embryo 

 within the body of the primary one. 



The gregariniform larva, or locomotive ovum, of the Di- 

 stoma tarda exemplifies the above propositions and plainly 

 illustrates the essential condition of its Parthenogenesis, or 

 power of propagating ( sine concubitu/ 



Were any one to assert that the development of the cer- 

 cariform larva from one of these secondary germ-masses was 

 altogether distinct from the development ab ovo, by being ( a 

 multiplication of cells by a process of continuous growth*,' 

 w r e could regard such assertion in no other light than as a 

 purely arbitrary one, and as betraying a want of appre- 

 ciation of what must be effected when the primary impreg- 

 nated germ-cell subdivides itself into the secondary ones 

 in order to form the germ-mass. 



Reverting again to the gregariniform larvae, let us sup- 

 pose that besides the cells of the germ-mass of the ovum of 

 the Distoma which have perished as such to form the larval 

 integument, others had coalesced to constitute special en- 

 velopes of parts of the unchanged germ-mass, and to form 

 canals leading from those ovarian envelopes outwards : if 

 any of such included germ-masses set on foot the processes 

 of forming an embryo, such embryo would be excluded 

 ( per vaginam' as in the Aphis, instead of bursting through 

 the integument as in the gregariniform entozoal larva. 



* The reverse of this idea is so obvious, that the veritable correspond- 

 ence between ovulation and gemmation could not escape the notice of 

 the equal observer of nature. Dujardin alludes to the common recog- 

 nition of it in his 'Memoir on the Infusoria': ' Les gemmes, les 

 bourgeons qu'on voit se detacher du corps des zoophytes peuvent encore 

 etre compares jusqu'a un certain point avec les germes detaches de 

 1'ovaire des animaux plus parfaites." Annales dps Sciences Nat. 1838, 

 p. 290. 



