1SS6.] OVUM OF LEl'IDOSIREN. 287 



formed in the interior of the ovum itself and has not an extrinsic 

 oricin. 



kolessnikow ', however, concludes his paper on the Teleostean 

 and Amphibian ovum with the statement that the yolk is chiefly a 

 product of the follicular epithelium, which behaves in this respect 

 like a gland. Apart from this fact, the follicular cells do, however, 

 play an important part in supplying the ovum with nutriment in 

 most Vertebrata ; but this is not in the form of yolk, which is sub- 

 sequently elaborated in the ovum itself. In Elasmobranchs Balfour 

 has noted ^ that certain of the cells become larger than the others, 

 and a;)parently communicate within the substance of the ovum 

 into which they pour their contents. Heape ^ has described some- 

 thing very similar in the ovarian ova of the Mole, as also have 

 Lindgren \ von Sehlen \ and Virchow ". The part which the 

 enlarged follicular cells of Elasmobranchs play in the nutrition of 

 the ovum is not the direct formation of yolk. Balfour has shown 

 that the yolk originates within the protoplasm of the ovum, and is 

 not transferred thither from the follicular cells. The reasons for this 

 statement are : — (1) that the yolk-spherules first of all appear in the 

 deeper portions of the yolk and not in the more superficial layers, 

 as they would naturally do if they were passed into the interior of 

 the ovum ; (2) that there is no trace of yolk-particles in the follicular 

 cells themselves. 



Iwakawa's observations on the yolk in the egg of Triton lead to a 

 similar conclusion. Gotte states that in Bombinator the yolk- 

 spherules first make their appearance in the peripheral layers of the 

 ovum, but is inclined to think that they are formed within the 

 substance of the ovum, and that they are not extrinsic in origin. 

 Brock {loc. cit. p. 560) quotes and confirms Gegenbaur'^ to the 

 effect that the yolk-spherules in Teleosteans originate by the fusion 

 of minute yolk-particles, and that this formation takes place in the 

 peripheral layers of the ovum ; only exceptionally was the neigh- 

 bourhood of the germinal vesicle the seat of yolk-formation. In 

 Mammalia, according to Balfour the yolk is formed in the peripheral 

 layer of the ovum. 



Lepidosiren therefore appears to be remarkable in that the yolk 

 is often formed in the follicular cells and transferred thence to the 

 ovum. Seeing that this is the rule in many of the lower Inver- 

 tebrata, the occurrence of this method of yolk- formation in Lepido- 

 siren would appear to be the retention of an ancestral character. 



' Loc. cit. 



^ Quart. Journ. Micr. Sci. 1878. 



3 Quart. Jourii. Micr. Soi. Feb. 1886. 



* Arch. f. Anat. u. Phys., Anat. Abtheil. 1877. 



' Arch. f. Anat. u. Phys., Anat. Abtheil. 1882. 



^ Arch. f. mikr. Anat. vol. xsiv. (1884). 



' " Ueber den Bau und die Entwickelung der Wirbelthiereier mit partiellei- 

 Dottertheihing," Miiller's Ai-ch. 1861, p. 405. Gegenbaur describes (p. 524) 

 a peculiar fatty degeneration of the follicular cells which serves ro loosen them 

 from the ovum when the latter is ready for extrusion. Possibly this is 

 to be referred to a trace of yolk-formation comparable to that described 

 in this paper. 



