l6o THE GERM-PLASM 



use of it as an example of gemmation in which two germinal 

 layers are primarily concerned, we can leave this question aside. 

 It is at any rate true that in the Polyzoa parenchymatous cells, 

 as well as a certain ectoderm cell, take part in each process of 

 budding. We must therefore assume that the determinants of 

 the species cannot be contained altogether in one cell as blasto- 

 genic idioplasm, as in the case of the Hydrozoa, but that a 

 number of them — including those for the muscles, endothelia, 

 blood-corpuscles, and perhaps those for the sexual organs also 

 — are supplied to certain mesoderm cells of the parent. The 

 development of sexual cells renders it necessary that those cells 

 from which they arise shall also contain germ-plasm ; and the for- 

 mation of the epidermis of the bud. which results to some extent 

 on purely mechanical grounds, presupposes the existence of deter- 

 minants for the ectoderm in the epidermic cells of the parents. 



The disintegration of the determinants, which is necessary 

 before budding can take place, is obviously, however, of a very 

 different kind from that which occurs in embryonic develop- 

 ment. Seeliger, indeed, has called attention to the fact that the 

 ontogeny which results from gemmation is a much shorter 

 process than that which occurs when an embryo and larva are 

 formed. In the former case, not only are the whole series of 

 stages of segmentation and development of a free-swimming 

 larva absent, but even in the later periods of development none 

 of the stages in embryogeny and in gemmation exactly corre- 

 spond to one another. Without following out these two pro- 

 cesses in detail. I should be inclined to explain them in general 

 by assuming that the groups of latent supplementary deter- 

 minants, with which certain cells are provided in the course of 

 embryogeny, contain combinations of determinants different 

 from those whicli lead to the development of the embryo. 



C. — Tiinicata 



The tixed Ascidians usually multiply very extensively by gem- 

 mation, and thus give rise to stocks, the individual persons of 

 which are more or less closely connected with one another. 



We owe our detailed knowledge of the process of budding 

 in the genus Clavelina to the researches of Seeliger.* The 



* O. Seeliger, ' Zur Entwicklungsgeschichte der Ascidien ; Eibildung u. 

 Knospung von Clavelina lepadiformis', Sitzungsber. d. Wien, Akademie, 

 15(1. 85, 1882. 



