ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 489 



white mice in the search for the explanation of the reversal. He has 

 published a preliminary notice of his results. There is a special 

 envelope of covering cells within which the cells of the embryo 

 proper undergo their development. (This is perhaps the stage 

 described by Spec — supra — in the guinea-pig, as a vesicle with a 

 clump of cells at one end.) The embryo-cells lie at one end, separate 

 into the two primitive layers, and become united with a support 

 formed by a knob of cells attached to the uterine wall. This knob 

 is not used in the construction of the embryo. The mass of ecto- 

 derm-cells becomes hollow, and the cavity increases in size. In the 

 ectodermal cells limiting it, the ectodermal organs of the embryo are 

 developed according to the typical processes in other mammalia. 



Embryology of Mice.* — The observations of Selenka and Kupffer 

 on the development of mice have been critically reviewed by V. Hensen. 

 He does not accept their views as to the gastrulation, or that the 

 formation of the cavity bounded by the ectoderm is the gastrula 

 development. Selenka attributes the reversal of the germ-layers to 

 the proliferation of the ectoderm-cells ; but Hensen maintains it to 

 be due to the invagination of the mass of cells forming the embryo- 

 germ. The ectodermal cavity in Arvicola does not correspond, as 

 would seem natural, to the amniotic cavity of the guinea-pig ; for an 

 amnion is subsequently developed in its interior. (Does not this 

 rather indicate that Hensen's homologizing the ectodermal cavity in 

 the guinea-pig with the amniotic cavity is erroneous, and that it is 

 really the same as the ectodermal cavity described by Selenka and 

 Kupffer?) Finally Hensen discusses briefly the position of the 

 germinal disk in guinea-pigs, and compares it with that of rabbits. 



Embryology of Rodents.t — J. Paladino gives the following resume 

 of his results, and, as they have a slight priority of publication over 

 recent German papers they deserve especial attention. 



The whole cylinder formed during the first developmental stages 

 of certain rodents is the embryo, and it is implanted on the decidual 

 new formation by the caudal extremity. This is proved especially 

 by the fact that it is this part from which the allantois arises. The 

 cylinder, and the vascular portion of the decidual new formation, are 

 continuous, and so remain throughout gestation by means of the 

 vessels falsely called omphaloid. The decidua forms not only the 

 placenta, but also the first envelope around the embryo, — the chorion, 

 falsely so called. (This is in direct contradiction to the latest opinions 

 of Hensen.) Between the embryo and the decidua is a large space, 

 filled at first with blood, which Paladino thinks is probably pro- 

 duced by the metamorphosis of the granulosa cells discharged from 

 the Graafian follicle along with the ovum (!). 



Origin of the "Vertebrate Mesoderm. J — G. Eomiti discusses His's 

 view that the mesoderm has a double origin, in part from the primitive 

 streak, and in part from independent cells, which His calls para- 



* Arch. f. Anat. u. Physiol., Anat. Abth., 1883, pp. 71-5. Cf. Science, 

 i. (1883) p. 407. 



t Arch. Ital. Biol., ii. (1882) pp. 363-7. Cf. Science, i. (1883) p. 525. 

 X Ibid., pp. 277-9. 



