ass 
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DR. RANSOM, ON GASTEROSTEUS LEIURUS. 3 
food, or formative, yelk of ripe eggs, and is remarkable for 
its solidity. 
The yelk-sac is formed in very young ova, and is separable 
in those measuring =4,” in diameter. It is then easily 
recognised by the little button-shaped processes or villi 
attached to the outer surface of its germinal segment sur- 
rounding the micropyle. The finely-dotted structure was made 
out in the yelk-sacs ofeggs, measuring ;4,”. The prevailing 
opinion among physiologists seems to be that the yelk-sac is 
formed at a much later stage of the development of the egg. 
That this is not so in osseous fishes is easy to prove ; it remains 
to be seen whether they are exceptional in this respect.* 
The peculiar structure of the yelk-sac, which is the same 
in the youngest ova as in the ripest examined, offered facili- 
ties for inquiring into its mode of growth.t+ 
Thus, in young eggs measuring +4,” in diameter, the yelk 
sac had 24,000 dots to the inch, while in ripe eggs measuring 
=~” but 11,000 dots to the inch were present, the egg 
increasing about six times in diameter ; the interval between 
the dots but little more than doubling. As the size of the 
dots had increased but little it is certain that during growth 
there must have been an increase in the number as well as in 
the size of the dots, which we may speak of as structural 
elements of the yelk sac. ri 
Again, in young eggs about +4,” to ~;” diameter, the 
number of buttons on the outer surface is on an average of 
five countings 80. In ripe ovarian ova there are on an 
average 207 buttons on the outer surface of the yelk-sac. It 
is, therefore, not conceivable that the mode of growth which 
has hitherto been accepted for cell-walls and yelk-sacs can be 
* Owen (‘Comparative Anatomy and Physiology of Vertebrates,’ vol. i, 
pp. 593 and 594) speaks of a yelk-membrane, distinct from the dotted sac, 
or “Ectosac,” and he refers to figures of Dr. Allen Thomson’s, in- 
correctly attributing them to me. I cannot confirm the view thus stated, 
and believe the figures to be wrongly interpreted. The ‘‘ Ectosac ” is a true 
yelk-sac. It does not, as Owen states, receive its villi after the escape of 
the ovum from the ovisac, nor does the escape take place, as he also seems 
to think, before the ovum is ripe. The interesting question of the homo- 
logies of the yelk-sac in Vertebrates is probably not yet settled, but the 
dotted sac of osseous fishes is certainly homologous with the structureless 
yelk-sac of Batrachia, and, like it, is early formed in the ovisac, and lies in 
direct contact with the mass which cleaves after impregnation. 
+ The membrane may briefly be described as composed of very fine con- 
centrically-arranged amine. ach layer is marked by dots arranged alter- 
nately so as to mark the angles of lozenge-shaped spaces. In the separate 
lamine the dots correspond to each other in such a manner that they form 
lines or strie, vertically placed in the substance of the yelk-sac; and 
whether examined on the outside or inside of the yelk-sac, are equal in size 
and distance from each other. (See Fig. 5.) 
