W. K. BROOKS ON THE GENUS SALPA. 201 



the mesoderm is two-layered, so that a parietal and a visceral layer may 

 be distinguished, there is no homology between these layers and the 

 bounding walls of the co3lom of the enterocoelomata." 



After reviewing all the literature on the subject, he gives as the 

 general result of his studies the statement (p. 622) that " the body cavity 

 of the ascidians lies between the two primary germ layers and must be 

 regarded as a blastocoel, which would be identical with the segmentation 

 cavity if this were not temporarily obliterated during gastrulation by the 

 contact of the ectoderm cells and endoderm cells." 



"While the salpa-embryo is very complicated and unfavorable for 

 studying this question, my own observations, which have already been 

 described, seem to show that the body cavity of salpa is, like that of 

 clavelina and distaplia, a primary one, fundamentally identical with 

 the segmentation cavity, and that the mesoderm arises as free mesen- 

 chyma cells derived from the endodermal blastomeres. 



The body cavity of the salpa-embryo, Plate XXXV, 75, is identical 

 with the space between the somatic and visceral layers of follicle cells, 

 Plate XII, 15, and while there is a stage in which these two layers are in 

 contact, Plate X, Fig. 9, the follicular cavity is undoubtedly the same as 

 the cavity shown at 75 in Plate XI, Fig. 3, and this is the same as the 

 space which is shown in Plate X, Fig. 3, between the segmenting egg and 

 the follicle. 



In the chapter on the significance of the salpa-embryo I have given 

 my reasons for believing that this space is homologous with the segmen- 

 tation cavity of more normal tunicate embryos, and if this view be correct 

 the body cavity of salpa is not an enterocoel but a primary body cavity or 

 blastocoel. The mesoderm of salpa consists of free migrating cells, and 

 the chamber of the heart is part of the body cavity, so that these cells 

 pass through it ; and while salpa is a peculiarly unfavorable subject, my 

 observations are in complete accord with those which Seeliger and 

 Davidoff have made under simpler and more favorable conditions. 



No student of the embryology of tunicates has ever described any 

 trace of a series of body cavities, and Kowalevsky, the discoverer of the 

 coelomic pouches of amphioxus, failed to find anything comparable to 

 them in the tunicates, although the existence of a single pair of entero- 

 coels has been claimed by certain observers. Van Beneden and Julin 

 (Zool. Anzeiger, 4, 1881 ; Bull. Acad. Belg. (3) 7. 1884 ; Arch. Biol. 6, 1884) 

 believe that the anterior portion of the body cavity of ascidians arises as 

 a pair of gut-pouches, and that its mesoderm consists of a somatopleur 



