ALTERNATION OF GENERATIONS IN OOLIOLUM. 477 



Salpidae, consisting in both cases of an asexual generation followed 

 by a sexual generation. The only difference between the two would 

 be that the primitive buds produced by the asexual generation, 

 in Doliolum, multiply by fission, and that the sexual generation 

 develops in three different forms (L, .1/ and G). In this way also 

 the fact that the median buds entirely agree in structure with the 

 sexual generation would he explained (Fig. 245 />' and C). 



We have still to describe the development of the young buds after 

 their detachment from the primitive buds. According to Ul.tanin, 

 all these buds, whether lateral, median or sexual, develop more or 

 less in the same way, so that an account of the development of the 

 lateral buds will suffice. The young buds, immediately after abstric- 

 tion, resemble in structure the primitive bud and the ventral stolon, 

 consisting of an external layer of ectoderm and of the seven strands 

 mentioned above. We have already mentioned (p. 471) the different 

 views held by Grobben and Ul.ianin as to the significance of these 

 seven strands in connection with the further development of the bud. 

 We have as yet comparatively few statements as to the way in which 

 the young laid develops out of these seven primary rudiments, and 

 further investigation of this point is very desirable. 



The young bud (Fig. 246 A), from the time when it becomes 

 detached from the primitive bud, is a completely independent organ- 

 ism enclosed in ectoderm, but attached externally, like a parasite, 

 to the body of the " nurse " or to the foster form. This attachment 

 is brought about by means of a thickening of the ectoderm (ec). In 

 the youngest buds observed by Uljanin, the body was already 

 elongated, and the organs, as compared with the seven cell-strands, 

 had already changed their positions. The dorsal side can now be 

 distinguished by the presence of the large rudiment of the nervous 

 system (n), while, on the ventral side, the pericardial rudiment (j>) 

 is to be observed. Between these two, the pharyngeal rudiment (ph) 

 can be seen, while the paired genital rudiment, forming a common 

 cell-mass, has shifted into the neighbourhood of the point of attach- 

 ment of the laid. The ectodermal invagination (<■/), behind the 

 nervous system, represents the rudiment of the atrium. This is one 

 of the principal points in which Qljanin's description differs from 

 that of Gbobben. According to the latter author, the atrium arises 

 from paired rudiments (the strands k in Fig. I'll') already present in 

 the primitive bud. At the two sides of the body, the muscle-plates 

 (///). lying in close contact with the ectoderm, have extended con- 

 siderablv. 



