OCR Text |
Show 18S6.] OVUM OF LEPIDOSIREN. 287 formed in the interior of the ovum itself and has not an extrinsic origin. 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 subsequently elaborated in the ovum itself. In Elasmobranchs Balfour has noted2 that certain of the cells become larger than the others, and apparently communicate within the substance of the ovum into which they pour their contents. Heape3 has described something very similar in the ovarian ova of the Mole, as also have Lindgren4, von Sehlen5, and Virchow8. 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 Gegenbaur7 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 neighbourhood of the germinal vesicle the seat of yolk-formation. Iu 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 Invertebrata, the occurrence of this method of yolk-formation in Lepidosiren would appear to be the retention of an ancestral character. 1 Loc. cit. 2 Quart. Journ. Micr. Sci. 1878. 3 Quart. Journ. Micr. Sci. Feb. 1886. 4 Arch. f. Anat. u. Phys., Anat. Abtheil. 1877. 5 Arch. f. Anat. u. Phys., Anat. Abtheil. 1882. 6 Arch. f. mikr. Anat. vol. xxiv. (1884). 7 " Ueber den Bau und die Entwickelung der Wirbelthiereier mit partieller Dottertheilung," Miiller's Arch. 1861, p. 405. Gegenbaur describes (p. 524) a peculiar fatty degeneration of the follicular cells which serves to 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. |