392 



TUNICATA. 



Mention must now be made of cells which, in the later stages of 

 cleavage, are found in large numbers in the yolk, near the point at 

 which the germ-disc lies on it, and which may be called yolk-cells 

 (Fig. 184, dz). SALENSKY, who traced back these cells to follicle- 

 cells that had immigrated into the yolk, has named them the yolk- 

 kalymmocyteK. Since, however, as we shall see below (p. 395), these 

 yolk-cells take part in the formation of the intestinal wall, we are 

 inclined to regard them as blastomeres belonging to the entoderm-part 

 of the germ-mass. We here have a repetition of the conditions found 

 in the meroblastic egg of the Vertebrata, in which also yolk-cells (to 

 be considered as entoderm) are said to take a similar part in the 

 formation of the intestinal gland-cells. 



For information as to the formation of the germ-layers in Pyro*<.nu,n 

 we are dependent entirely on SALENSKY'S statements. The embryo, 



after a number of cell- 

 divisions, lies on the yolk 

 as a prominence composed 

 of uniform polygonal cells 

 which are irregularly distri- 

 buted. This prominence 

 soon becomes bilaterally 

 symmetrical, the largest 

 mass of cells collecting in 

 the posterior half of the 

 germ-disc, so that the pos- 

 terior slope is more abrupt 

 than the anterior (Fig. 185). 

 According to SALENSKY, the 

 separation of the germ-layers 



takes place through delamination, the most superficial cell-layer (ec) 

 first becoming arranged into an epithelium (ectoderm) ; the mass that 

 remains (the meso- entoderm) then undergoes a similar transformation, 

 the lowest layer, that in contact with the yolk, becoming separated 

 as the intestinal epithelium (entoderm). Between the ectoderm and 

 the entoderm the mesoderm extends, being greatly developed in the 

 posterior half of the germ-disc while, in the anterior half, it is want- 

 ing or else is represented merely by a few cells (Fig. 186 A and ). 



Taking into account the process of formation of the germ-layers in the 

 meroblastic eggs of the Vertebrata, we may perhaps be allowed to conjecture 

 that in Pyrosoma also the separation of the germ-layers is not an actual 

 delamination, but an invagination or infolding of the posterior edge of the 

 germ-disc, such as, for instance, occurs in the Selachians. 



FIG. 135. Median section through a germ-disc 

 of Pyrosoma (after SALKNSKY). ch, cavities 

 of the chorda ; do, food-yolk ; dz, yolk-cells ; 

 ec, ectoderm ; mn, meso-entoderm ; h, pos- 

 terior ; v', anterior. 



