236 Prof. W. Salensky o?i the 



XXVIII. — On the Development o/Pjrosoma. 

 By Prof. W. Salensky*. 



Since Huxley's celebrated investigations we have learnt to 

 distinguish two periods in the development of Pijrosoma^ 

 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- coXonj. 

 The discovery of this peculiar method of development has 

 led to the view that in Pyrosoma we have a case 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 " inner follicle-cells " described 

 by Kowalewsky, for which I now propose the more general 

 name " 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 Pijrosoma 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 tlie first place, 

 Kowalewsky did not altogether pay sufficient attention to it 

 in his investigations, and^ secondly, because the mesoderm- 



* Translated from the ' Biologisches Ceutralblatt,' Band x. Heft 8, 

 June 1, 1890, pp. 225 et seq. 

 t From KoXvfjifxa, a veil. 



