337 
In Scyllium!) among other results?) it is found that at the 
critical period the sexual ducts are practically established, the sex of 
the organism is defined internally (by ovary or testis), as well as ex- 
ternally, the retina is becoming pigmented, the first rudiments of scale- 
development are present, and, for the first time there is a be- 
ginning of an internal yolk-sac containing yolk, and 
thenorvery soon after— immediately afterwards if not 
then, foragain there is a slight individual variation — 
yolk is found in the gut-cavity. 
This discovery, then, was the starting point for my little inquiry 
into the history and fate of the yolk-sac and its contents. 
a) Seyllium. 
In these pages it is intended that the account to be given shall 
be as brief as possible. In the main only the results obtained will 
be recorded, the description of the observations reine reserved for 
fuller treatment elsewhere. 
Regarding the earlier stages of the history of the yolk-sac and 
its contents in Elasmobranchii there are practically no new facts to be 
recorded. During the earlier part of the development the yolk is ab- 
sorbed by the embryo solely by means of the blood-vessels of the sac *), 
and this remains as the method of its absorption, until what I have 
termed the critical period. As a matter of course the yolk plates are 
not directly taken up by the blood, but, as H. E. ZIEGLER, HOFFMANN, 
myself and others have insisted, the yolk is prepared for absorption 
by the curious yolk-nuclei or merocytes. 
There are two points about these latter which demand some dis- 
1) The length of the embryo is then about 32 mm, but allowance 
must be made for individual variation. 
2) The points already noted, with one minor exception for Scyl- 
lium relating to the thymus, also hold for Scyllium, Lepidosteus and 
some others, and the new facts also hold for Raja. The thymus of 
Scyllium is, however, not exceptional; for, as J. SCHAFFER has demon- 
strated, in Petromyzon it is connected with the gill-epithelium in the 
adult fish. 
3) In Vol. II of the Comparative Embryology p. 53 Batrour writes: 
“Nutriment from the yolk-sac is brought to the embryo partly through 
the umbilical canal, and so into the intestine, and partly by means of 
blood-vessels in the mesoblast of the sack”. As a matter of fact these 
two modes are adopted at different periods of the development, and the one 
first described is initiated in Scyllium embryos of 32 mm or thereabouts 
and in embryos of Raja batis of 71 mm or thereby. 
Anat, Anz, XI]. Aufsätze. 24 
