976 SUMMAEY OF CUREENT RESEARCHES RELATING TO 



affect tlie whole form of the produce ; this inheritance-substance 

 (idioplasm) is contained in the germinal vesicle of the ovum and in the 

 spermatozoon, both of which have the significance of nuclei ; the first 

 nucleus formed by them is to be regarded as hermaphrodite ; from it 

 there arise all the nuclei of the complete creature in unbroken series, 

 and they are, therefore, representatives of both the producing organ- 

 isms. The special activities of the smallest particles of which they 

 are composed are the conditions of the multiplication-phenomena of 

 the cells and of their growth. The typical forms of organs and of 

 organisms are the consequence of definite combinations of cell-divisions 

 and cell-growths and these are ruled by the nuclei. 



The writer discusses the chemical constitution of the nuclei, and 

 concludes with the aphorism that there are many cells in the organism 

 which are either embryonic in character, or may be regarded as 

 such. 



Development of the Opossum.* — Having succeeded in keeping a 

 large number of North American opossums alive for a lengthened 

 period, Prof. E. Selenka obtained material for a detailed study of their 

 development. Keserving the main embryological results, which will 

 doubtless throw much light on the development of the placental 

 mammals, he communicates a few preliminary notes of which the 

 following seem the most important. 



1. Spermatozoa, There are in each sperm-cell two spermatozoa 

 remaining long united, even within the vagina, where they at length 

 violently separate in consequence of the increased rapidity of their 

 tail-vibrations. 



2. Fertilization occurs in the lower end of the oviduct five days 

 after copulation. Gestation lasts eight days, and then the young are 

 transferred to the pouch. 



3. Segmentation is intermediate between partial and total. While 

 division is proceeding a nutritive yolk collects at the aplastic pole 

 of the ovum, and this, at first quite outside the ectoderm, is in three 

 days covered in by the adjacent ectoderm and mesoderm cells, though 

 never coming within the umbilical vesicle. Remains of this yolk 

 persist till three days before birth. 



4. The ova, which exhibit a most rapid growth and development, 

 are at first scattered in the uterus, and on the fourth day when 

 fertilization has begun, they become loosely fixed to the uterine 

 epithelium. 



5. The number of embryos varies from nine to twenty-seven, but 

 in the marsupium there were never more than six young opossums. 



Karyokinesis in Segmentation of Axolotl Ovum, j — Prof. J. 

 Bellonci describes the nuclear changes in the segmentation of the 

 ovum of axolotl. The nucleus is at first ellipsoidal with a notch at one 

 side, and with a relatively small quantity of chromatin. As karyo- 

 kinesis begins, chromatin filaments form the familiar ball of coils ; 

 some achromatin threads appear within, the star-shape is then 



* Biol. Gentralbl., v. (1885) pp. 294-5. 



t Arch. Ital. de Biol., vi. (1885) pp. 52-7 (1 pi.). 



