628 SUMMARY OF CURRENT RESEARCHES RELATING TO 



stem-form ; the generation of the ascidiozooid was, however, inter- 

 calated between the sexually formed solitary form, and the first 

 generation of ventral buds. The cycle of the composite Ascidians 

 had a somewhat different origin, for it sprang from a bud which 

 appeared after the free-swimming stem-form became fixed : the various 

 differences which we now observe only appeared later in its history. 



It is probable that the pelagic Tunicates and the Ascidiae are 

 only connected by a very old larva-like stem-form, which was endowed 

 with the power of multiplying asexually. This stem-form was 

 distinguished from the Appendiculariae by the two arterial passages 

 fusing into a dorsal cavity, which opened to the exterior by an un- 

 paired orifice. 



In his second chapter the author considers the developmental 

 history of the Ascidians in its relations to the theory of the germinal 

 layers ; he concludes that there is a fundamental difference between 

 mesenchym and mesoblast, but that it is purely morphological, and 

 that no genetic conclusions are to be based on its consideration. The 

 mesoblast arises primitively from diverticula of the archeuteron ; 

 secondly, from paired mesoblastic mother-cells which lie near the 

 blastopore, and give rise to the mesoblastic sacs which inclose the 

 secondary coelom ; thirdly, in the Tunicata it arises directly from the 

 lateral walls of the archenteron. The formation of the first two 

 kinds of mesoblast is associated with the formation of a new secondary 

 ccelom ; in the Tunicata, however, there is no secondary coelom, but 

 the primary, being narrowed not only by the peribranchial space, but 

 also by mesenchymatous connective-tissue-cells, and by an inner 

 mantle of cellulose (in some forms) must be regarded as a true 

 pseudocoel. 



In the third chapter the genetic relations of the Tunicate phylum 

 are discussed, and the following table is given : — 



Ascidian trunk 



Asc. Social. Botryllidse 



Polyclim*da3 "j 



ISynascidiae 

 Asc. Simpl. / ^^'^Didemnidse J 



Doliolidfe 

 PyrosomatidfB Anchinia 



Salpida) 



Appendiculariae 



Salpine trunk 



