Development of Pyvoiomn. 237 



question for the Tuiiicates in general cannot be regarded as 

 having been exhaustively worked out. 



1. The KaJymocytes of the Ovum of Pyrosoma and their 

 Function in the Development of the Cyathozooid. 



So far as I am aware, Kowalewsky was the first to observe 

 the occurrence of kalymocytes in the ovum of Pyrosoma. 

 Kowalewsky terms them " inner follicle-cells," but recog- 

 nizes their homology with the so-called test-cells of the 

 Ascidians. He also described the mode of origin of these 

 cells with perfect accuracy, and shows that they are nothing 

 else than follicle-cells which have separated from the follicle- 

 wall and wandered into the space between the latter and the 

 surface of the yolk. From Kowalewsky's figures we can at 

 once see that the kalymocytes (" inner follicle-cells," Kow.) 

 difi'er in form and structure from the true follicle-cells. As a 

 matter of fact these cells differ so much from the blastomeres, 

 not in form and structure only, but also in the way in which 

 they are affected by staining-reagents, that, even with a low 

 power, they can be very easily recognized in stained prepa- 

 rations. 



The kalymocytes appear in the ovum of Pyrosoma at a 

 very early stage, and are to be observed in tolerably large 

 numbers even before the commencement of segmentation. 

 With reference to their origin, I can completely confirm 

 Kowalewsky's statements ; different stages in the separation 

 of these cells are very easily made out in sections. As regards 

 the structure of these cells, however, Kowalewsky is not 

 altogether accurate. This is explained by the fact that 

 Kowalewsky underestimated the role of the kalymocytes, 

 and therefore pays them less attention than they actually 

 deserve. The structure of the kalymocytes is very charac- 

 teristic, although their form varies according to the place in 

 which they are found. They are to be met with in different 

 parts of the ovum — immediately beneath the wall of the 

 follicle, in the interior of the yolk, or between the blasto- 

 meres, and they are everywhere distinguished by a diff'erent 

 shape, corresponding with their situation. The cells found 

 at their place of origin have a primitive shape, which we may 

 regard as typical. They are pyriform, tapering at one pole 

 and widened at the other. Each cell contains within its 

 tapering portion a nucleus, which, owing to the readiness with 

 which the protoplasm takes a deep stain, is not very con- 

 spicuous in coloured preparations. The nucleus is vesicular, 



