718 PROFESSOR W. C. M'INTOSH AND MR E. E. PRINCE ON 



nuclei, originating as independent segregations of active protoplasm, like the nuclei which 

 arise endogenously in the Molluscan ovum, as Professor Ray Lankester was the first to 

 recognise. In Crustacean ova such nuclei have long been known, though in Oniscus 

 it is noteworthy that Bobretzsky affirms their blastodermic origin and subsequent 

 migration ; but this view is not generally accepted. Weissman, too, imagined that in 

 the ova of Dipterous insects such structures arise de novo, and without genetic relation 

 to nuclei already existing ; but later researches lend little countenance to this opinion, 

 and Weissman has abandoned his contention. Kowalewsky has described in the yolk- 

 matrix of the Annelidan ovum scattered nuclei, endogenously formed and afterwards 

 collecting superficially, especially beneath the blastoderm ; they are at first few in num- 

 ber, but show rapid increase, and are especially abundant about the time of exclusion. 

 He regards the nuclei of the " intermediary layer " in the Teleosteans as originating from 

 those of the entoblastic (yolk-) cells. The appearance of free nuclei in the region outside 

 the embryonic area in the chick, as described by Rauber (No. 133, p. 570), is a further 

 instance of such extra-embryonic nuclear bodies, and the nuclei in the Teleostean 

 periblast may have a like origin.* The fact that they differ in shape from the 

 spherical nuclei of the disc — being generally more or less elliptical, and often of 

 larger size (PL II. fig. 6, n) — points to a non-blastodermic origin. Kupffer speaks of 

 their differentiation, and of delicate contours which appear round them resembling hexa- 

 gonal figures, in Clupea (No. 87, p. 205). Lereboullet observes that they are large and 

 granular in JEsox, and along with the matrix in which they lie, they "come from an- 

 other source " than the protoplasm and nuclei of the disc (No. 93, p. 494). Balfour, again, 

 comes to the conclusion, while leaving their origin an open question, that there is no 

 evidence of their derivation from pre-existing nuclei in the blastoderm (No. 10, p. 109). 



In the living Teleostean ovum it is difficult to watch the actual formation of these 

 nuclei ; but Kupffer describes with some detail the appearance in Clupea of clear spots 

 of protoplasm which grow from a speck-like particle to a body 5-6 jx in diameter (op. 

 cit., p. 201), and E. van BENEDENt is no less decided in affirming that these nuclei 

 arise " par voie endogene " simultaneously in the periblast. We have noted that in 

 the egg of the cod, towards the end of the first day, the periblast shows only minute 

 granules scattered through its translucent protoplasm. The nuclei | are few at first, 

 and close to the edge of the disc, as if some of them had escaped by " hernia." At other 

 parts of the periblast clear vesicles and minute granules occur. Observations do not 

 strongly support the view that the nuclei of the periblast migrate from the archiblast, 

 but probably they arise in the periblast itself, and it may be that the activity in the disc 

 proper stimulates similar activity in the periblast, just as a limited area of irritation in 



* Ryder regards the " nuclear zone" as homologous with this germinal wall in the chick, and it is certainly note- 

 worthy that the nuclei in the latter (the " white yolk nuclei ") are most abundant below the thickened periphery of the 

 blastoderm, and become the nuclei of cells which enter the germ. t Belg. Acad. Sc, No. 6, June 1876, p. 1202. 



X Kingsley and Conn {op. cit., p. 199) observed in the dinner the formation of cells round these nuclei on the 

 surface of the yolk; but it seems, according to Mr G. Brook, that Mr Kingsley has since altered this view {Trans. Roy. 

 Soc. Edin., 1887, p. 224). 



