EMBRYONIC FISSION IN OYOLOSTOMATOUS POLYZOA. 17 



larity in the succession of the blastomeres, which are, in fact, 

 inextricably entangled among the follicle-cells. They are not 

 necessarily in contact with one another, but may be separated 

 from one another by ingrowths of the follicle-cells, so that in 

 most preparations it is almost impossible to count the number 

 of the blastomeres, or to distinguish all of them from the fol- 

 licle-cells. An excellent idea of the general relation of the 

 blastomeres to the follicle-cells (or nuclei) may be obtained by 

 referring to some of Salensky's figures of Sal pa, as his pi. x, 

 figs. 12, 13. The only diflference that I can point out between 

 Crisia and Salpa, as regards the relation of the blastomeres 

 to the follicle-cells, is that in Crisia the follicle is somewhat 

 larger relatively than in Salpa, and that the blastomeres 

 occupy only the central region of the follicle instead of filling 

 up most of that structure, as in Salpa. Remains of the 

 follicle-cavity may still be detected in some of these stages. 



In fig. 6 (which is connected with fig. 5 by numerous pr .- 

 parations, forming a perfectly continuous series, in my pos- 

 session) the tentacle-sheath has increased in size so much as 

 to fill up nearly the whole of the ovicell. The follicle has 

 largely increased in size, and now forms a sub-spherical knob, 

 projecting freely into the cavity of the tentacle-sheath. This 

 stage is a perfectly constant and easily recognised one. 



The scattered blastomeres have at last come together to form 

 a small but compact embryo, in some of the nuclei of which 

 karyokinetic figures are discernible. There is no trace of the 

 differentiation of germ-layers in the embryo, which consists 

 simply of a small rounded mass of undifferentiated embryonic 

 cells, or rather of a continuous mass of protoplasm, containing 

 nuclei scattered through it without any attempt to arrange 

 themselves in definite layers. 



At the distal end of the embryo is a clear part of the follicle 

 which contains small nuclei. This is apparently a constant 

 feature of the stages near this one ; but I have not been able 

 to make out its significance. 



Fig. 7 is not cut quite medianly, so that it does not show 

 that the attachment of the follicle to the tentacle-sheath is 



VOL. VI. 2 



