138 PROFESSOR E. VAN BENEDEN. 



toplasm of the axial cell is finely granular, and there is an 

 oval nucleus with well-defined outline. About this time, 

 when the embryo has elongated into a cylindrical or nema- 

 toid body (fig. 9), a germ appears in the central cell on each 

 side of the nucleus ; these germs are produced endogenously 

 and have a homogeneous nucleus. The end of the body at 

 which the blastopore was situated now rapidly developes 

 the cells, which ultimately form the polar velum ; the cells of 

 the head are differentiated from those of the trunk ; the dif- 

 ferentiation of the parapolar and caudal cells takes place at 

 a later period, generally after birth. At a given instant the 

 whole body clothes itself with vibratile cilia, the development 

 of which Prof. Van Beneden has not been able to trace, at 

 the same time the embryo elongates and becomes more 

 and more thread-like ; vacuoles appear in the protoplasm 

 of the axial cell and the number of germs increases. 

 The embryo at the time of birth is composed of the same 

 number of cells as are present in the adult, and the post^ 

 embryonic development consists entirely in the progressive 

 increase of the constituent cells of the embryo. 



II. Tlie Rhomhogenous Dicyemida. — These individuals, 

 which produce infusoriform embryos, are shorter and larger 

 than the preceding group ; the axial cell, too, is larger, and 

 terminates in front in a rounded end, so that the polar 

 cells are flatter and the head has a different shape to that 

 in the Dicyemidse producing worm-like embryos. The 

 germs are about double the size of those producing vermir 

 form young, and are not formed directly in the protoplasm 

 of the axial cell, but in a peculiar set of cells, which are 

 themselves produced in the cell of the endoderm ; these 

 latter cells, whose formation has not yet been followed, are 

 called the " germ-producing cells," or shortly, the " germ 

 producers" (figs. 4 and 5). They are always few in number, 

 varying from two to four or five, and have an oval nucleus, 

 which is in marked contrast to the spherical nucleus of the 

 germs ; the substance of the cells are finely granular. 

 Each germ producer gives birth to several generations of 

 daughter-cells, which are the germs of the infusorian em- 

 bryos, and are arranged concentrically round the mother- 

 cell (fig. 5). These germs are formed endogenously, and 

 the nucleus of the germ-producing cell takes no part in the 

 formation of the daughter-cells ; three or more spherical nuclei 

 appear simultaneously, round each of which a diflferentiated 

 layer of protoplasm can be distinguished, which is less 

 granular than the ])rotoplasm of the primitive germ producer. 

 Occasionally a radiating appearance may be seen in the 



