EMBRYONIC FISSION IN CYCLOSTOMATOUS POLYZOA. 209 



are seen disconnected from the primary embryo. The con- 

 strictions indicate the limits of as many future embryos. 



The *' primary embryo " contains^ distally, an irregular 

 cavity. It is difficult to be sure of the exact arrangement of 

 the embryogenic processes ; but in some cases at least it is 

 evident that the distal end of the primary embryo has the 

 form of an irregular cup, the processes forming the wall of the 

 cup, from which they become free at their ends. The irre- 

 gular cavity seen in fig. 11 is part of the cavity of the cup. 



Towards the ends of the processes an ectodermic layer 

 becomes clearly differentiated ; while, in the centre of each of 

 the swellings indicating a future embryo, a small group of 

 inner-layer cells can, in some cases, be clearly distinguished. 

 The ectoderm of the processes is continuous with the outer 

 nucleated layer of the primary embryo, and with the similar 

 layer immediately lining its distally-placed cavity. The inner- 

 layer cells of the secondary embryos are continuous with the 

 inner nuclei of the more solid, proximal half of the primary 

 embryo. But these nuclei and the protoplasm surrounding 

 them are not throughout clearly differentiated from the outer 

 layer of nuclei. I am, however, inclined to suppose that the 

 somewhat triangular, clear mass of protoplasm at the proximal 

 end of the primary embryo, containing actively dividing nuclei, 

 is the region which gives rise to the inner-layer cells. This 

 region can generally be distinguished with ease in ovicells at 

 this stage. 



The primary embryo consists of a mass of embryonic cells 

 (or, rather, nuclei embedded in continuous protoplasm) which 

 are obscurely differentiated into outer and inner cells (or 

 nuclei) . The whole function of this embryo is to act as an 

 embryogenic organ, or producer of secondary embryos, and it 

 possesses no structures which can be described as its own 

 organs. 



At its proximal end, the primary embryo is budding off 

 nuclei which migrate into the protoplasmic reticulum, where 

 they become indistinguishable from the rest of the nuclei 

 of that reticulum. I have been unable to make out the 



