1886. ] 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 sub- 
sequently elaborated in the ovum itself. In Elasmobranchs Balfour 
has noted? 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. 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 
statemeut 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. Gétte 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. 
1 Loe. cit. 
2 Quart. Journ. Mier. Sci. 1878. 
3 Quart. Journ. Mier. Sci. Feb. 1886. 
+ Arch. f. Anat. u. Phys., Anat. Abtheil. 1877. 
5 Arch. f. Anat. u. Phys., Anat. Abtheil. 1882. 
® Arch. f. mikr. Anat. vol. xxiv. (1884). 
7 “Ueber den Bau und die Entwickelung der Wirbelthiereier mit partieller 
Dottertheilung,” Miller’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. 
