341 
In emb. 67 (86 mm) the external yolk-sac, when sectioned, was 
seen to be almost empty of yolk, there were very few merocytes 
within it and these were rapidly degenerating. Some of the cells of 
the hypoblastic lining contained yolk-plates, which they appeared to 
be digesting. 
In emb. 68 (90 mm) the external yolk-sac was quite flaccid and 
empty. There was no yolk at all within it. Its epiblastic and meso- 
blastic walls were, as in even earlier stages, undergoing retrogressive 
changes, and its hypoblastic lining was rapidly breaking 
up and suffering complete atrophy. 
Thus, the external yolk-sac shrivels up, as is, indeed already 
known, and its internal hypoblastic lining is broken up 
in situ and absorbed. 
The latter fact is surely of importance. 
Such is the history of the merocytes, yolk and yolk-sac in Scyl- 
lium, and, so far as it has yet been followed out, Raja agrees with 
the dog-fish in all that has just been recorded. The story is remark- 
able, but the whole comparative history is wonderful and astonishing, 
when the fate of the merocytes in Lepidosteus is included in the 
narrative, and when one takes a wide survey of the history of the 
yolk, the yolk-sac, and yolk-cells or merocytes in the Vertebrate 
series *). 
b) Lepidosteus. 
In this Ganoid it is as easy, as in the other cases already dealt 
with, to establish a connection between the initiation of degeneration 
in the transient nervous apparatus and very important and deep- 
reaching changes in the embryo itself. When the proper occasion ar- 
rives, it will be demonstrated that a critical period in the development 
is as marked as in Scyllium or Raja. In slight respects this differs 
from that of the latter two Elasmobranchs. Here these differences 
call for no detailed consideration, and it may be merely remarked, 
that they mainly concern the absence of scale-rudiments and the non- 
present time my notes of measurements have not been tabulated, and in 
the collection there are still a number of embryos, whose sizes and yolk- 
sac dimensions have not been taken, but the general impression in my 
mind from the examination of more than a dozen embryos of upwards of 
68 cm is, that the rate at which the yolk-sac is absorbed is subject to 
individual variation. 
1) Vide “On certain Problems of Vertebrate Embryology”. Jena, 
G. Fischer, 1896. 
