Development o/Pyrosoma. 239 



the other cells which form the wall of the enteron. The 

 movement of the yulk-kalymocytes towards the germinal 

 disk, liowever, 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 Fyrosoma 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 tiiem 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 iirst 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 tlie 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 eyathozooid. 



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- 



