240 Prof. W. Salensky on the 



appears ; at tlie same time the latter assume a polygonal form, 

 in consequence of the reciprocal pressure of adjoining cells, 

 and grow more and more lii<^e the blastomeres. Theconstitution 

 of the protoplasm of tlie kaljmocytes presents a more lasting 

 characteristic, which distinguishes these cells from the blasto- 

 meres. This is, however, not constant, and in time this character 

 too disappears. AYe have remarked above that vacuoles appear 

 at a tolerably early ])eriod in the protoplasm of the kalymocytes; 

 their number continually increases with the progress of 

 development, so that in the stages at which the embryo con- 

 sists of several hundred cells the protoplasm of the kalymo- 

 cytes appears as a perfectly transparent viscid mass, traversed 

 in different directions by a finely granular network of threads 

 of the original substance. In consequence of this the kaly- 

 mocytes of these stages appear paler in stained preparations 

 than was formerly the case. Simultaneously with this the 

 constitution of the protoplasm of the blastomeres also under- 

 goes a change, in that it loses its previous finely granular 

 structure, and appears more and more homogeneous and 

 trans))arent. 



From this we see that the changes of the blastomeres go 

 hand in hand with tiiose of the kalymocytes. In both cases 

 the result is a clarification and liquefaction of the protoplasm. 

 If we consider at the same time that the differences in size 

 between the blastomeres and kalymocytes gradually fade 

 away, it follows that in the final stages of segmentation the 

 two kinds of cells, kalymocytes and blastomeres, of which the 

 segmented nucleus consists, must look precisely alike. As a 

 matter of fact, if we examine a section from the later segmen- 

 tation-stages of the oosperm of Pyrosoma, we find that the 

 embryo consists of a large number of cells of precisely similar 

 structure. Kalymocytes are no longer to be distinguished 

 from blastomeres. Since in the subsequent stages of develop- 

 ment all the cells of the embryo take an equal part in the 

 formation of the cyathozooid, we arrive at the conclusion tiiat 

 the cyathozooid is formed from two different elements : — 

 (i.) from the derivatives of the fertilized egg-cell — the blasto- 

 meres, which throughout the whole of the animal kingdom 

 alone play the part of formative elements ; and (ii.) from the 

 non-fertilized elements — the kalymocytes, which unite with 

 the former and assume the role of the formative elements. 



The processes of development in the ovum of Pyrosoma 

 which I have just described, in spite of their peculiarity, are 

 not entirely unique in the series of developmental phenomena 

 which have been discovered in the animal kingdom in recent 

 times. The nearest ai)proacli is made by those desciibod by 



