236 
Prof. W. Salensky on ike 
XXVIII. — On the Development of Pyrosoma. 
By Prof. W. Salensky *. 
Since Huxley’s celebrated investigations we have learnt to 
distinguish two periods in the development of Pyrosoma 7 
viz. : — (i.) The evolution from the fertilized ovum of a nurse- 
like form, which Huxley termed the “ Cyathozooid ; ” (ii.) The 
formation by a species of budding of a group of four Ascidian- 
shaped individuals, the Ascidiozooids of Huxley, which must 
be regarded as the parents of the entire Pyrosoma- colony. 
The discovery of this peculiar method of development has 
led to the view that in Pyrosoma we have a ease of meta- 
genesis occurring in the ovum. A few years after the appear- 
ance of Huxley’s monograph on Pyrosoma , the investigation 
of the development of these interesting forms was undertaken 
by Kowalewsky, who increased our knowledge in several 
respects, and especially as regards the finer histological rela- 
tions of the embryonic processes. The segmentation, forma- 
tion of the germinal layers, and organogeny of the cyatho- 
zooid, as also of the ascidiozooids themselves, were very 
minutely described by Kowalewsky ; and it seemed at the 
time as if the new observer in the same field would have but 
few fresh discoveries to make. Nevertheless subsequent 
progress in the science of comparative embryology has brought 
certain questions to the front which in Kowalewsky’s work 
are scarcely touched upon. Two such questions, which are 
of general interest, I shall attempt to answer in this short 
paper, so far as my own investigations permit me to do so. 
The first of these concerns the u inner follicle-cells ” described 
by Kowalewsky, for which I now propose the more general 
name u kalymocytes ” f. The part which these cells play in 
the development of the cyathozooid of Pyrosoma has hitherto 
been a puzzle ; the remarkable behaviour of similar cells in 
the development of the Salps may suffice as a reason for 
undertaking a fresh examination of Pyrosoma and of the 
metamorphoses of the kalymocytes of the ovum of Pyrosoma 
in particular. The second question which I intend to discuss 
in these pages refers to the origin and metamorphosis of the 
mesoderm ; and I have selected it because, in the first place, 
Kowalewsky did not altogether pay sufficient attention to it 
in his investigations, and, secondly, because the mesoderm- 
* Translated from the 6 Biologisches Centralblatt/ Band x. Heft 8, 
June I, 1890, pp. 225 et seq . 
f From KdXvfjLfxa, a veil. 
