an 
335 
Many embryologists have, indeed, devoted attention to the early history 
of the yolk, but most of them have, possibly from lack of sufficient 
material, been content to break off their researches, soon after the 
merocytes were developed, or, when the circulation on the yolk-sac 
was established. 
One investigator!) only, so far as I am aware, has recognised, 
in his attack of the problem, that something more than the history 
of the yolk and yolk-cells during the earlier periods of the development 
was urgently required. 
H. VircHow’s results are so recent, and are incorporated in the 
pages of such widely-read journals, that here, where space is valuable, 
any detailed reference may be omitted. 
Moreover, an opportunity for a fuller account of other previous 
work on the subject may soon arise in the pages of another journal. 
It is not without some hesitation that the results of my own work 
are now recorded in brief form. But, as, in the natural course of 
things, they will probably find a place in a memoir?) bearing a title 
of a different character, as a small item in a piece of work, out of 
which the observations on the yolk-sac have unavoidably and naturally 
arisen, it has seemed fitting to put those results also on record in a 
Separate publication. For the line of argument in the memoir men- 
tioned below my results on the yolk-sac are not without their own 
importance, and, as historically they arose out of the work on the 
transient nervous system, it may not be so very much out of place 
to include them in part II. The reader of my recent work on Raja 
batis*) may remember that a certain stage of the development was 
described as the “critical stage”. It corresponded to the period when 
the embryo was making for the adult form, in other words, when the 
embryo-skate was first recognisable as a skate, and when it had at- 
tained a sufficient degree of independence‘) to be able to set about the 
1) Hans Vırcnow, Der Dottersack des Huhnes. Festschr. R. VırcHow, 
Berlin 1891, p. 223—8353. — Das Dotterorgan der Wirbeltiere. Zeitschr. 
f. wiss. Zool., Bd. 58, Suppl. — Das Dotterorgan der Wirbeltiere. Arch. 
f. mikrosk. Anat., Bd. 40. — Dotterzellen und Dotterfurchung bei Wirbel- 
tieren. Verhandl. Anat. Gesellsch. Wien 1892, p. 209. 
2) It is intended that these results should form a chapter of Pt. II 
of my work on “the transient nervous system in certain Ichthyopsida”, 
and, therefore, they will probably appear in the Zool. Jahrb., Morph. 
Abteil. 
3) J. Brarp, The History of a transient Nervous Apparatus in certain 
Ichthyopsida. Pt. I, Raja batis. Zool. Jahrb., Morph. Abteil., Bd. IX. 
4) In his well-known memoir on Lopadorhynchus (Z, wiss. Zool., 
