6o SECOND GENERAL MEETING. 



the layer showing the characters of an endoderm evolves as an 

 ectoderm and vice versa. Thence, two hypotheses are possible : 

 either the invagination has been reversed, each layer keeping its 

 normal histological characters, or the invagination takes place in 

 the normal way, the layers having interchanged their histological 

 characters. Let us examine which of these is the best. 



The last one is supported by nothing. Neither in my mind 

 nor in the literature can I find the least reason to admit such a 

 disguise of the layers. 



However, Professor Perrier from Paris — without indeed even 

 attempting to give an explanation of the disguise of the layers — 

 has raised against my theory an objection which, were it of value, 

 would come in support of this hypothesis. According to Professor 

 Perrier, \hQ principle of coujiexions of Is. Geoffroy Saint Hilaire 

 obliges us to consider as endoderm the inner layer and as ectoderm 

 the outer one throughout the animal kingdom, be the other 

 characters as they will. But, we have seen that in the siliceous 

 Sponges, the layer which is inside in the adult is outside in the larva, 

 and that which is inside in the larva is outside in the adult. 

 Therefore the principle is necessarily wrong in one of the cases, 

 either in the larva, or in the adult. Hence it has no decisive value 

 and gives no support to the theory \ 



The principle of connexions is indeed a very valuable one, but 

 it has been misunderstood by Professor Perrier. The real connexion 

 of the layers depends not upon their situation inside or outside of 

 the gastrula but upon their continuity at the equator of the blastula 

 or at the blastopore of the gastrula. Invagination is a secondary 

 process which depends on physiological causes and has nothing to 

 do with the primitive connexion of continuity. 



On the contrary, it is easy to understand that the invagination 

 could have been inverted by various causes, perhaps not very 

 significant in themselves, such as an increase or decrease of the 

 rigidity, of the pressure, of the rate of growing, in one or in another 

 part of the blastula. Some well-known experiments of Herbst, 

 Driesch, Gurwitsch have demonstrated that a few thousandths 

 of a salt of lithium mixed to the water where Pluteus-embryos of 

 Sea-urchins are nursed, or even a temperature some degree higher, 

 is enough to cause the invagination to lack or even to reverse itself 

 outwards when already produced in the normal ^\■ay. 



Another fact comes in support of my idea. One can often 

 observe the blastula of some calcareous Sponges invaginating 

 itself, now in one way then in the reverse way, as if trying different 



^ I beg leave to reply in advance to an objection which is likely to be raised by my 

 honourable opponent. It is of no use to say that the larvce of the siliceous Sponges have 

 been modified by Ceenogenesis or such other factor of evolution. If the principle of 

 connexions, such as it is understood by Professor Perrier, has been once defective, it can 

 be so twice; if it has been mastered by Cxnogenesis, it can have been checked by the 

 unknown cause of the inversion of the invagination in the Spongida. 



