Calcispongiae in the Animal Kingdom. 259 



In opposition to these statements, the sexual products of the 

 Acalephje originate from the exoderm^ in the Siphono]5hora 

 according to Keferstein and Ehlers*, in Cordylopliora ac- 

 cording to F. E. 8chulze (/.c. p. 36), and in Hydra according 

 to Kleinenberg (/. c. pp. 30, 32). 



In the Sponges the origin of the sexual cells could not 

 hitherto be investigated in connexion with this question, 

 because the fundamental construction of their body, of the two 

 formative membranes, and the homology of these with the 

 exoderm and entoderm of the Acalephte, as also with the two 

 germ-lamellfe of the higher animals, had not been recognized. 

 When I first demonstrated this homology, as a matter of 

 course the question from which of the two lamellfe the sexual 

 cells originate could not but acquire great importance for me. 

 I have discussed this question in detail in my third chapter, 

 and have finally arrived at the result that hoth forms of sexual 

 cells are with great jprohahility to he derived from the entoderm. 

 Unfortunately, however, I cannot assert this with as much 

 certainty as could be desired, and I must still leave the 

 opposite possibility open. 



8. The Biogenetic Fundamental Principle. 



" Ontogenesis is the brief and rapid recapitulation of 

 phylogenesis, governed by the physiological functions of 

 transmission (reproduction) and nutrition (adaptation). The 

 organic individual, dm-ing the rapid and brief course of its 

 individual development, repeats the most important of those 

 changes of form which its ancestors have passed through 

 during the long and gradual course of their paleeontological 

 development in accordance with the laws of transmission and 

 adaptation." It is with these words that, in my general 

 history of development f, I have expressed the theory of the 

 causal nexus of ontogenesis and. phylogenesis ^ or hiontic and 

 phyletic development^ upon which it is my firm conviction 

 that the whole inner comprehension of developmental history 

 depends, and which I therefore placed at its head as the 

 hiogenetic fundamental principle. Witli this first " funda- 

 mental principle of organic development " the whole de- 

 scendence-theory is inseparably united ; the two stand or 

 fall together. This has been shown in a most admirable 

 manner by Fritz Miiller, in his ingenious phylogeny of the 

 Crustacea |. 



* Zoologische Beitriige, 1861, p. 2. 



t Generelle Morphologie, 1860, Bd. ii. pp. 0, 300 ; Natiirl. Schcipf ungs- 

 gesch. 3rd edit. 1872, p. 362. 

 I Fur Darwin, 1864. 



17* 



