50 



PHYLUM TUNICATA (UKOCHORDA). 



much more elaborate, consisting of eight pigmented structures 

 on the dorsal side of the ganglion. 



The development takes place within the follicle described above and 

 is remarkable for the fact that the follicle cells play an important part in 

 transferring nourishment from the maternal organism to the embryo. 

 They do this in two ways : (1) the follicle cells, on the side of the embryo 

 which future development shows to be ventral, proliferate and form a 

 thick mass which soon assumes a cavernous structure. This is the pla- 

 centa (Fig. 43). Maternal blood passes through its spaces and it is con- 

 tinually detaching small masses which, charged with nutritive matter, pass 

 into the vascular system of the embryo and afford nourishment for the 

 growing tissues. ' (2) The follicle-cells in other parts of the follicle pro- 

 liferate and migrate in amongst the blastomeres, with which they become 

 inextricably intermingled, so that it is difficult if not impossible to dis- 

 tinguish between them. These cells are called calymnocytes (gonoblasts). 

 Their fate is much disputed, but there can be but little doubt that they, 

 like the plac.-ntal tissues, afford nourishment to the growing embryo. 



FIG. 42. Stages in the cleavage of Salpa democratica-miicronata (after Salensky from Kor- 

 schelt and Heider). In A and B the oviduct is open, in C it is closed, and the oviducal and 

 follicular epithelia have combined to form the follicle, a atrial epithelium ; b shortened 

 oviduct ; c follicle-epithelium ; / blastomeres ; z calymnocytes. In C, c points to the rudi- 



nic-iit ni the placenta. 



Many views have been held as to the nature of the calymnocytes. Sal- 

 ensky is of opinion that they actually form the embryo, and that the em- 

 bryo of Salpa is a follicular bud and not a true embryo. Brooks thinks 

 that for a time they actually form the embryonic organs but are eventually 

 replaced and probably consumed by the blastomeres. 



Meanwhile the embryo has been growing and projecting more and more 

 into the atrial cavity. The follicle cells and the atrial epithelium over it 

 become thin and eventually rupture so that the embryo lies free in the atrial 

 cavity attached only to the placenta (Fig. 43). 



The formation of the layers is impossible to follow, but the organs gradu- 

 ally make their appearance in the confused mass of embryonic and follicular 

 cells. With regard to them it is only possible to note a few points here. 

 The pericardium is. according to Salensky, developed from the mesoderm ; 

 but Korotneff asserts that it arises as a diverticulum of the pharynx, which 

 is perhaps the more usual way in Tunicata. The ganglion is by most 

 observers described as arising from a mesodermal mass which becomes 

 secondarily penetrated by an endodermal diverticulum of the pharynx. 

 It is at first hollow and its cavity communicates with the pharynx. 



