44 SUMMARY OF CURRENT RESEARCHES RELATING TO 



The cells appeared to Kollmann true segmentation cells. They never 

 arise from yolk-spberes, which are always disintegrated near the embryo. 

 A radial arrangement of the protoplasm round about the nuclei was dis- 

 tinctly observecT, and cell-complexes undoubtedly result. They are not 

 undefined elements arising from the yolk or from the primitive lymph of 

 the segmentation cavity. 



The yolk-spheres never have nuclei, nor is the yolk penetrated by 

 protoplasmic filaments. They consist simply of nutritive material and are 

 gradually assimilated. Not only the yolk, but all contents of the ovum 

 have to be modified and reconstituted before forming part of the embryo, 

 and on this fact Kollmann lays considerable emjihasis. 



Ovarian Ovum of the Dipnoi.* — Mr. F. E. Beddard finds in the ovary 

 of Lepidosiren two kinds of ova which follow a different course ; one is of 

 the ordinary type, the other consists of a number of distinct cells, and 

 appears to have no germinal vesicle ; in Ceratodus the second kind of ovum 

 is found, but seems to be very rare. 



Alternation of Generations in Mammalia.! — In reference to the re- 

 productive relations of Praojnis, of which a rejDort has already been given,J 

 Dr. H. V. Ihering communicates some suggestive notes on alternation of 

 generations. In Praopus eight embryos resulted from a single germ, in 

 Lumhricus trapezoides a double embryo is constant, in all groups twins 

 may occur from one ovum ; — the polar bodies are morphologically nothing 

 less than abortive germs ; — in fact, the origin of multiple embryos from 

 a single ovum is the primitive condition, the development of only one is 

 secondary and adaptive. Now if this be pressed to its logical conclusion, 

 one would be forced to the paradoxical conclusion that the Praopus, for 

 instance, brings forth grandchildren, and that the mother of twins from one 

 ovum is really their grandmother. The categories are evidently insufiicient, 

 and von Ihering proposes the following revised scheme : — 



I. HoLOGEXOUS DEVELOPMENT (Hackel's hypogcnesis). The fertilized ovum 

 developes with or without metamorphosis into a siwjle individual. 

 II. Mebogenous development. The fertilized ovum developes into two 

 or more individuals, which 



(A) return directly to the parent form and mode of reproduc- 

 tion (Temnogenesis) ; 

 or (B) exhibit an antithesis of diversely reproducing individuals 

 or generations (Metagenesis, or Alternation of Genera- 

 tions) ; 



(a) calycogenesis, in Salpse and Medusae ; 

 (6) paedogenesis, in Cecidomyise ; 



(c) heterogenesis, in which either both generations 

 reproduce sexually, or one or several multiply 

 parthenogenetically. 



Experimental Investigation of Fertilization. § — Prof. E. Hertwig re- 

 ports some of the results of experiments carried on by himself and his 

 brother Prof. 0. Hertwig as to the effect of different reagents on the process 

 of fertilization. The reagent discussed is chloral hydrate ; the elements 

 experimented on were those of Strongylocentrotus lividus. 



* Zool. Anzeig., ix. (1886) pp. 635-7. t Biol. Centralbl., vi. (1886) pp. 532-9. 



X See this Journal, 1886, p. 765. 



§ Anat. Anzeig., i. (1886) pp. 11-16. Cf. also SB. Jenaisch. Geeell. f. Med. u. 

 Naturw., 1886. 



