Larval Theory of the Origin of Tissue. 197 



only locally on the walls of the avchenteron in the intervals 

 between the diverticula (primitive arapuUge) *. 



In the mesenchyme of sponges the cells have been subjected 

 to fewer changes, and they preserve their ancient amoeboidal 

 forms unaltered. The comparatively great change in the evo- 

 lution of the group probably took place after the transfer of 

 the principal seat of assimilation from the endoderm to the 

 mesenchyme. This transfer possibly occurred during the 

 genesis of Sycones and other higher forms. 



The researches of Saville Kent among Protozoa have shown 

 that the collar and flagellum are feeding-organs, and we must 

 imagine them as having a similar meaning in the internal 

 cavity of Ascones, the lowest forms of sponges. 



When we consider the whole series of transformations of the 

 ovum it becomes apparent that it is at first an autotemnon 

 having the Amoeba stage well and clearly developed. The 

 ovum develops parallel with the spermatocyst through the 

 period of division of the nucleus into two parts, the masculo- 

 nucleus and the feminonucleus. We have tried, in common 

 with some other authors, to show that the masculonucleus 

 is probably thrown off in the polar globules during a process 

 of agamic division of the nucleus, and that these are the homo- 

 logues of the masculonuclei excluded from the spermatocyst 

 after having been transformed into spermatozoa. 



The remarkable essays of Professor Ed. Van Beneden on 

 the bisexual nature of the nucleus are the only embryological 

 writings which produce the proofs of this hypothesis in illustra- 

 ted form. This author (" Fecond. Maturat. de I'Qiluf/' Archiv. 

 de Biol. tom. vi. 1883) advances precisely similar views to 

 those of Dr. Minot, and shows the phenomena of fecundation 

 and the double composition of the maritonucleus in a series of 

 remarkably clear illustrations. Van Beneden claims to be the 

 discoverer of the bisexual composition of the nucleus of the 

 ovum, and refers to his paper of December 1875 (Bull, Acad. 

 de Belg. vol. xl. 1875) as containing the first statement of his 

 discovery. Though not pretending to forestall the judgment 

 of those better qualified to decide the merits of these claims, 

 we find that Professor Van Beneden was the first to announce 

 the basal facts of the bisexual theory, but that he did not give 

 all of the essential conditions of the phenomena of conjugation 

 between the male and female parts of the nuclei in his first 



* Von Lendenfeld (Austral. Sponges, Proc. Linn. Soc. N. S. Wales, 

 vol. ix. pi. iv.) describes Homodenna sycandra as having these cells 

 equally distributed all over the endoderm as well as in the sin"-le 

 ampullfe. 



