ENTOZOA. 



113 



nucleus ; and the surrounding granules are aggregated about it by 

 the formation of a yolk membrane, which compresses them into a 

 spherical mass, the clear fluid being, as it were, filtered through, as 

 the membrane contracts, into the interspace between the germ-mass 

 and the cliorion. In this state the egg quits the parent in the Asc. 

 mystax. 



The subsequent changes observed by Dr. Nelson in this species 

 accord with those which have been described by Barry, in the mam- 

 miferous ovum *, and by Sieboldf, Kolliker|, and other observers, in 

 the impregnated ova of the Entozoa and other invertebrate animals. 

 The nucleus of the primary germ-cell {Jig- 48, a) first divides. Its 

 division {Jig. 49.) is followed by that of the germ- cell itself (^^. 50.) ; 

 and the two " secondary germ-cells" {fig. 51, h, b) thus established, 

 48 4\) 50 51 52 5i 



64 



Development of Ascarls acuitiinata. LXXXV. 



recede to opposite ends of the germ-yolk. Dr. Nelson has observed 

 that these germ-cells revolve in circles, each appearing to mould its 

 portion of the germ-yolk into a spherical form ; and he considers the 

 subsequent division of the germ-yolk to be a mechanical eifect. § By 

 its division, two germ-yolks, each with its nucleated germ-cell, result 

 {fig. 52.). The division of the germ-cell takes from five to ten 

 hours ; the subsequent one, of the yolk, not more than half an hour, 

 in the Asc. mystax. By the repetition of this process, as in figs. 53, 

 54, 5o, tlie number of derivative germ-cells increases in the geome- 

 trical ratio of 4, 8, 16, 64, 128, 256, &c., until the whole egg appears 

 to be filled by opaque spherical granules {fig. 56.), each, however, 

 having its share of the original impregnated germ-cell. A membrane 

 is then formed around the germ-mass. 



In the ova of Nematoidea, many of which are vivipai'ous, the 

 embryo is developed by two diiferent modifications of the diffusive 

 process by multiplication of germ-cells from the primary centi-al one : 



* XXVII. p. 307. 



t LXXXV. 

 I 



X LXVI. 



§ XCI. 



