28 SIDNEY P. HARMER. 



(most Gyranolaemata; cf. especially Seeliger, No. 33). There 

 is no sufficient reason for supposing that a young zooecium 

 consists of anything but ectoderm and mesoderm. The Ecto- 

 proct larva may be considered morphologically as a young 

 zooecium containing a potential "brown body" (the remains 

 of the purely larval organs)^ and it is not unreasonable to 

 suppose that the structures found in the larva of the Cyclo- 

 stomata, developed as it is by a process of budding, are com- 

 parable with those which are found in a zooecium. 



We arrive, therefore, at the provisional conclusion that the 

 inner layer of the Cyclostome embryo is more likely to repre- 

 sent the mesoderm than the endoderm of the larva. 



There can be no doubt that, on the assumption that my 

 account of this process is in the main correct, the development 

 of Crisia takes place in a manner to which there are few 

 known parallels. 



The most frequently quoted case of embryonic fission is that 

 of Lumbricus trapezoides, in which, according to the 

 statements of Kleinenberg (20), the embryo normally divides 

 into two complete embryos at the gastrula-stage. In some 

 abnormal cases, however (1. c, p. 217), a single embryo is first 

 formed ; and this gives rise to one or more embryos produced 

 as buds on the margin of its mouth. The segmentation of the 

 egg is described as being much less regular than in other species 

 of Lumbricus, in which no embryonic fission takes place. 



An equally striking case of the same kind had previously 

 been described by Busch (7), in Chrysaora. In only a few 

 cases does an egg develop into a single embryo. In the other 

 cases, the embryo gives rise to one or two buds, apparently at 

 the gastrula-stage ; the buds becoming free larvae, and deve- 

 loping fresh buds. Not only does Busch claim to have 

 followed the whole process in an isolated individual, but he 

 states that each time that the water in which the young larvae 

 were kept was changed, two thirds or so of the embryos were 

 thrown away, and that this loss in number was compensated 

 for, by the next day, by the geramiparous habit of the larvae 



