368 L. DONCASTER. 



begin to separate again into two layers^ a somatic layer next 

 to the body-wall, and a thin splanchnic layer surrounding the 

 endoderm, and in this way the coelomic cavities reappear. 

 At first there are two small cavities in the head, and two 

 large ones extending the whole leugth of the body, those in 

 the head being separated from those behind by a transverse 

 septum. These may be regarded as the same as those 

 formed early in embryonic life, when the head cavities are 

 separated ofl: from the rest of the coelom. After the two 

 posterior ccelomic cavities have reappeared, and extend 

 through the whole length of the body, important changes 

 take place in the region of the genital cells, which result in 

 the division of the coelom into an anterior and posterior part, 

 and so give rise to the condition found in the adult. 



Up to this time the genital cells have lain embedded in 

 the mesoderm a little behind the abdominal ganglion, and 

 pressed close to the alimentary caual ; they are oval in shape, 

 and their longest direction coincides with the long axis of 

 the body (fig. 17). At about this time, however, they begin 

 to change their position, and gradually come to lie with their 

 long axes placed transversely to the length of the animal. 

 They then move slowly across the ccelomic cavity until they 

 reach the body-wall on each side, when they again come to 

 lie end to end with their long axes in the same direction as 

 the length of the body. This process is gradual and takes 

 several hours, and while the large and conspicuous oval 

 nuclei travel across, protoplasmic connections can be seen 

 still attaching them to the wall of the alimentary canal, and, 

 when they approach the body-wall, also to the splanchnic 

 mesoderm (Pi. 20, figs. 20, 21). 



While they are traversing the ccelom the two of each pair 

 lie side by side close together, but not in contact, and 

 during their pi'ogress a transverse septum {se}). tr.) is formed 

 between them, so that when they arrive at the outer sides, 

 against the body-wall, a septum is left across the coelom, 

 dividing the body-cavity of each side into an anterior trunk 

 and a ])Obturior tail portion, as in the adult. The way in which 



