A Coutribut. to the Embryol., Life-history, and Classificat of the Dicyeiiiids. 79 



liave, during- its Rhorabogenic period, from 2 — 9 free nuclei, and during 

 the subsequent Nematogenic period, any odd number from 3 — 17 inclu- 

 sive. (Cf. p. 39.) 



10. The Infusorigen is a group of cells consisting, at one period 

 (fig. 109), of a peripheral layer of celLs partially enveloping a large 

 centrai celi. Its development from a single celi by a process of cleav- 

 age , and the epibolic growth of its peripheral layer , give ground for 

 thinking that the Infusorigen re}»resents aii individuai, comparable with 

 the Gastrula of a vermiform embryo. The peripheral layer may be 

 eompared to the ectoderm , the centrai celi to the axial celi , and the 

 »residuai nucleus« to the »centrai nucleus«. Germ-cells arise eudogen- 

 ously both in the centrai celi germogen) and in the axial endodermic 

 celi. For general considerations arisiug out of this Interpretation, con- 

 sult pp. 48—54, 59—61. 



1 1 . The two kinds of embryos produced by diphygenic individuals 

 arise from two distinct kinds of germ-cells, both of which originate, in 

 succession, in the Infusorigen. First in order, arise the germ-cells 

 destined to develop info infusoriform embryos. These infusorific 

 cells are comparatively large (.016 mm) and bave large nuclei (.01 — 

 .012 mm). They are set free in small numbers, one or two at a time 

 (p. 54) . At length the production of this kind of cells ceases, the infu- 

 soriform embryos escape, and nothing remaius of the Infusorigen except 

 the germogen (fig. 112, e) and a number of small loose cells (about 

 .008 mm in diam.) with small nuclei (.004^ — .<)05mm). These cells 

 multiply by division until they fili the larger portion of the axial cell, 

 and eventually give rise to vermiform embryos. The Rhombogenic 

 period is thus separated by a considerable iuterval (transitional period, 

 p. 41) from the Nematogenic. 



It very seldom happens that the development of vermiform embryos 

 begins hefore the infusoriform embryos bave ali escaped (p. 42 — 43). 



12. Developmental division (cleavage) and multiplicative division 

 are displayed in striking contrast in the history of diphygenic individ- 

 uals. Simple divisiou gives rise to from 2—8 scattered germ-cells; 

 cleavage produces a diploblastic group. the Infusorigen; infusorific and 

 vermific germ-cells are then produced by division ; and these in due 

 time udergo developmental division. The alternation between division 

 and cleavage is twice repeated. The structural unity of the Infusorigen 

 is attested by the cohesion of its cells. It has a developmental and a 

 reproductive period. (Cf. pp.40, 53.) 



13. No definite evidence of fecundation has been obtaiued ; but it 



