239 
Development o/Pyrosoma. 
the other cells which form the wall of the enteron. The 
movement of the yolk-kalymocytes towards the germinal 
disk, however, does not cease with the closure of the mesen- 
teric wall ; at any rate, some are always to be found beneath 
the latter after it is quite complete. 
The most important of all the varieties of kalymocytes are 
those of the nucleus — that is to say, those which come in con- 
tact with the nucleus from above. Since these cells stand 
later on in the most intimate relation to the blastomeres, we 
cannot describe them otherwise than in connexion with the 
segmentation. Since the investigations of Kowalewsky, it is 
well known that the ova of Pyrosoma are meroblastic. Before 
the first constriction appears the kalymocytes have already 
reached the nucleus. They range themselves on the upper 
surface of the latter and assume a variety of shapes. In 
stained sections, owing to the intensity of their colouring, 
they are very conspicuous. Some of them penetrate into the 
groove between the two blastomeres ; others lie on their 
upper surface ; while yet others actually bore their way into 
the interior of the blastomeres. The latter variety exhibit 
the most remarkable phenomena, which have so far hardly 
been observed in the case of the ovum of any other animal. 
The penetration of the kalymocytes into the interior of the 
formative portion of the oosperm can be very readily followed 
in the first stages of segmentation, even step by step. The 
significance of this peculiar phenomenon is, however, not so 
easy to see. The examination of several ova in the first 
stages of segmentation has led me to the conclusion that the 
occurrence of kalymocytes in the nucleus is confined to the 
very earliest stages of segmentation only ; after the nucleus 
has divided into four, the phenomenon entirely ceases. As 
regards the fate of the immigrant cells, my investigations 
enable me to state that these cells undergo no material 
changes within the nucleus. I therefore incline to the opinion 
that the kalymocytes remain in the nucleus for a short time 
only, and leave it again without suffering any structural 
change, and that we must not ascribe to the penetration of 
these cells into the nucleus any important influence on the 
development of the cyathozooid. 
During the subsequent stages of segmentation the kalymo- 
cytes congregate exclusively in the fissures between the 
blastomeres ; they preserve their primitive pear-shaped form 
for some time, and remain sharply distinct from the blasto- 
meres owing to their size. In proportion as the blastomeres, 
however, become continually smaller as segmentation proceeds, 
the difference in size between them and the kalymocytes dis- 
