488 SUMMARY OF CURRENT RE5EAJICHES RELATDia TO 



probable that they make a bole through the zona by which the egg 

 makes its exit. Spee has actually found, in one case, an empty 

 ruptured zona. This is an imj)ortant and interesting observation, 

 because the fate of the zona pellucida has not been hitherto determined. 

 Spee adds the suggestion that possibly the same protoplasmic processes 

 which serve to free the egg, also act to fasten it to the wall of the 

 uterus. 



As a continuation of Spec's paper, V. Hensen * describes an ovum, 

 soon after attachment to the uterine wall, found six days and twenty- 

 three hours after copulation. The egg (0* 13 x • 08 mm. in diameter) 

 lay in an open pit of the mucosa. It consists of a vesicle, with a mass 

 of cells on one side, therefore agreeing in structure with the latest 

 stage of the free ovum seen by Spee. Formerly Hensen considered 

 the mass of cells to represent the ovum, and the wall of the vesicle 

 to be an outgrowth of the epithelium of the uterus ; but he now with- 

 draws that interpretation, and accepts Schafer's view that the whole 

 is ovic. " The vesicle is therefore the single-layered primary chorion, 

 which is derived from the ectoderm, and is separated very early from 

 the embryo proper. In other mammals this separation does not 

 occur until after the formation of the amnion." The ectodermal 

 cells of the germ-mass of the embryo come to form a hollow, and this 

 hollow Hensen homologizes with the amniotic cavity of other 

 mammals. Of coui-se, therefore, it is bounded by the ectoderm, and, 

 beyond that, by the endoderm. The apparent reversal of the layers 

 is therefore due to the early development and peculiar position of 

 the amniotic cavity, inside the ovum. In conclusion, Hensen insists 

 upon the importance of showing that the histological value of the 

 germ-layers is really preserved, even in so unusual a form of develop- 

 ment as that of the guinea-pig. 



Activity of the Yolk during Impregnation.f — C. Kupflfer recalls 

 the active movement of a protoplasmatic prominence on the surface of 

 the ovum of Petromyzon, observed by A. Miiller, Calberla, and him- 

 self, immediately after the spermatozoon entered the yolk. He now 

 reports a similar observation on Bufo. In this animal several 

 spermatozoa enter the ovum ; but those that reach the egg a few 

 minutes after spawning are not able to pierce the egg-membrane. 

 One then sees little protuberances arise on the surface of tie yolk, 

 and stretch up the membrane. Opposite each protuberance are one 

 or two spern.atozoa, their heads towards the yolk. It appears as if 

 the yolk were actively striving to reach the spermatozoa. In a few 

 minutes the protuberances sink back. In both Bufo and Petromyzon 

 there appears this secondary act of impregnation after the male 

 elements (or element) have penetrated the yolk. 



Germ-layers and Gastrula of the Mouse.:}:— In some rodents the 

 germ-layers have apparently a position the reverse of that in other 

 animals. This fact has led E. Selenka to investigate the early stages of 



* Arch. f. Annt. u. Phvsiol., Anut. Abth., 1883, pp. 61-70 (1 pi ) 



t SB. Akad. Wiss. Miinchen, 1882, pp. 608-18 (1 fig.). 



; Biol. Ccutnilbl., ii. (1883) pp. 55J-8 (0 figs.). Cf. Scieuce, i. (1883) p. i07. 



