816 THE ANIMAL KINGDOM. 



but it is not known whether the monogenic and diphygenic individuals are or are 

 not different forms. The distinction between them, which was not recognised by 

 E. van Beneden, depends solely on the different modes in which their germs de- 

 velope. The primary Nematogen is most common in young Cuttlefish, the 

 Rhombogen in adult. 



In a young Dicyemid the axial endoderm cell contains in addition to its 

 nucleus several germ-cells. The first and second germ-cells are derived by division 

 of the nucleus of the axial endoderm cell and a separation of a part of its proto- 

 plasm round each of the nuclei thus formed. The remaining germ-cells are derived 

 by binary fission of the two first formed. In a Rhombogen each germ-cell present 

 developes as follows. First of all it divides : one half consists of a nucleus //^.r a 

 very small portion of protoplasm, if any at all. This half is clear, and remains in 

 statu quo as a * paranucleus.' The other half consists of nucleus and protoplasm, 

 and it proceeds to divide into a number of cells surrounding a single central cell. 

 The whole group is called by Whitman Infusorigen, a term used by E. van Beneden 

 as synonymous with Rhombogen. The central cell of the group, or germogen, pro- 

 duces fresh cells endogenously, i. e. by division of the nucleus within the germogen. 

 The cells of the group are set free, one or two at a time ; they are large with large 

 nuclei. The endogenously produced cells are similarly detached. All of them de- 

 velope into infusoriform embryoes. Finally, the germogen gives rise to a number of 

 small cells with small nuclei, which lie loose, i. e. not coherent in a group, multiply 

 by division, and fill the axial endoderm cell in great measure. From them originate 

 the vermiform embryoes, but the development of the latter rarely occurs before all 

 the infusoriform embryoes have escaped. The germogen cell, exhausted by the work 

 of fission, is finally reduced to a ' residual nucleus.' 



The axial endoderm cell of a primary Nematogen contains only its own nucleus 

 and germ-cells developing into vermiform embryoes; of a Rhombogen its own 

 nucleus -f one or more paranuclei equal in number to the Infusorigens present ; of 

 a secondary Nematogen, its own nucleus + a number of paranuclei + a number 

 of residual nuclei equal in number to the Infusorigens that have been resolved, + 

 many germ-cells developing or about to develope into vermiform embryoes. The 

 maximum number of Infusorigens produced is generally eight. Whitman is inclined 

 to believe that an Infusorigen is an individual equivalent to the Gastrula of the ver- 

 miform embryo. 



The infusoriform embryo has the form of a top or pear ; the broad end is the 

 head, the conical portion the tail. The head is non-ciliated, the tail ciliated. The 

 former consists of two dorsally-placed bodies, the refractile organs, which are two 

 modified ectoderm cells, and a single ventral organ, the urn. The urn consists of 

 a cover formed by four modified ectoderm cells, of a floor and sides formed by two 

 modified cells (? ectodermic), and of contents. The latter are four bodies, or at 

 first cells, containing granules which sometimes show ciliary motion, and hence are 

 probably spermatozoa (E. v. B.). They are produced by division of the nuclei of 

 the four cells, which in their turn are said to be derived from the two cells modified 

 to form the sides and floor of the urn. The tail of. the embryo is composed of 

 ciliated cells. It swims head foremost. Two views are possible as to its nature : 

 (i) That it spreads the race from one Cuttlefish to another. This view is supported 

 by van Beneden's assertion that it lives longer in sea-water than a fully-formed 

 Dicyemid, a fact denied however by Whitman. (2) That it is a male organism, a 



