30 C. 0. Whitman 



infusoriform and vermi form, are not found together, but each by 

 itself in diiferent iudividuals. 



These conclusions were confirmed by the researches of Ed. Van 

 Beneden, who followed the development of both forms with much more 

 care and success than had been done before. Further than this. Van 

 Beneden drew a sharp line of distinction between individuals hearing 

 infusoriform and those hearing vermiform embryos, designating the 

 former as Rhombogens, the latter as Nematogens. Beyond the 

 fact of their dimorphic progeny, already established by Kölliker , the 

 following dififerential characters were declared to exist : 



Rhombogens. Nematogens. 



1. Comparative ly short and thick. 1. Comparatively long and slender. 



2. Axial cell correspondingly broader, 2. Axial cell narrower, and tapering to 

 having a rounded termination in the a point in the head. 



head. 



3. Polar cells more fiat. 3. Polar cells thicker. 



4. Number ofectodermal cells variable. 4. Number ofectodermal cells Constant, 

 generally less than in Nematogens. and often greater than in Rhombogens 



of the same species. 



5. Germ-cells relatively small (0.012— 5. Germ-cells large (average 0.021 mm) 

 0.014mm); formedendogenouslyinthe formed endogenously in special cells 

 reticulum of the axial cell. (»germigens«) lodged in the axial cell. 



In view of the differences here ennmerated, and the concurrent 

 testimony of Kölliker and Van Beneden to the eflfect that the two kinds 

 of embryos are never found in the same individuai , it may , at first 

 sight, appear incredible that the Rhombogen and the Nematogen are 

 nothing more nor less than two consecutive phases in the same individ- 

 uai cycle of life. The presenta tion of the evidence on which this asser- 

 tion rests, will show that the distinctions before named, with some slight 

 modification, are not irreconcilable with it. 



The fact that these creatures can not be kept alive for more than 

 a few hours, precludes of course the possibility of tracing the transition 

 from one condition to the other by continued Observation on a single 

 individuai. 



The indirect character of the evidence as well as the peculiar nature 

 of the phenomenon itself calls for a detailed statement of observations 

 and the inferences drawn from them. 



Van Beneden's studies led him to think it probable that Rhombo- 

 gens and Nematogens are heterogeneous forms , permanently distinct 

 from each other. But he remarks : «I do not know what determines the 



