A Contribut. to the Embryol., Life-liistory, and Classificat. of the Dicyemids. ój 



a division of labor by exalting, in one group of cells, functions that are 

 to a greater or less extent complementary to those exalted in other 

 groups. A cbange of a similar nature oceurs wben au organisni — 

 Sacculina for instance, — removed froni tbe struggle for existence by a 

 parasitism tbat makes food and safty attainable witbout exertiou , un- 

 dergoes wbat is called retrogressive metamorpbosis. The process of 

 simplification brings tbe animai to a stnte in wbicb tbe dissipation of 

 force is minimized and tbe reproduetive power enormously increased. 

 It matters very little wbetber we cali tbis process retrogressive or pro- 

 gressive , tbe result is evidently a great improvement in so far as tbe 

 propagation of tbe species is concerned. Tbe same result migbt be 

 reacbed in a more direct way by arrest of development. In wbat otber 

 ligbt can we regard*Distomatous S p o r o e y s t s and Re di a e ? 



It is doubtless exceptional tbat development sbould stop sbort 

 witb tbe Gastrula stage, and tbat every celi sbould sbare tbe reprodue- 

 tive power ; but since tbe number of cells in tbe active eujoymeut of 

 tbis power may vary in different animals from one to many , it is cer- 

 tainly not incredible tbat it sbould include ali, particularl}' if tbe organ- 

 isni, balting at a ver}' early stage, begins to reproduce under circum- 

 stances of an extremely favorable kiud. 



Wben we reflect tbat in some animals (38, 50) tbe production of 

 germ-cells is tbe peculiar work of tbe ectoderm ; andfurtber, tbat in 

 tbe bistorical development of an animai not only may tbe same organ 

 assume different functions at different times, but also tbe same function 

 may sbift from one set of cells to anotber (37), tbe fate of tbe peripberal 

 cells of tbe Infusorigen ceases to be any serious objection to tbe view 

 tbat tbey represent an ectoderm. 



Altbougb tbis view is admissible and apparently sustained by tbe 

 figures of botb Van Beneden and myself , tbe ((uestion of its teuability 

 must be decided by tbe development of tbe Infusorigen. Van Beneden 

 did not observe tbe earliest stages of tbis development, and hence very 

 naturally inferred tbat tbe peripberal cells seen in bis figs. 15, and 20 — 

 24, PI. 1, and in my figs. lOS — 110, PI. 5, arise witbin tbe germogen 

 in tbe same manner as later generations of germ-cells. According to bis 

 Statements, tbere is at tbe outset a large celi (germigène), witbin wbicb 

 tbere arise, independeutly of tbe nucleus, successive generations of cells 

 tbat are disposed concentrically around tbe motber celi. Tbis concentric 

 arrangement is tbus explained : »Quand les germes nés dans l'intéri eur 

 d'un germigène ont atteint un certain volume, ils se portent vers la sur- 

 face; le protoplasme qui séparé les germes se contraete, il s'amasse 



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