564 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Wall of Yolk-sac, and Parablast of the Lizard.* — Dr. H. Strahl finds 

 that the yolk sac of reptiles presents many resemblances to that of birds ; 

 the yolk-sac is at no period a vesicle equally thick in all its parts and com- 

 posed of two simple layers ; the mode of growth of the endoblast appears to 

 be peculiar, for it does not widen out as a sj^ecial epithelial membrane, but 

 its cells are found around the yolk. 



In agreement with Kupffer, the term parablastic is applied to the cells 

 which lie beneath the endoblast, after the developmentof the three germinal 

 layers ; their parablastic cells may be seen even during the cleavage period, 

 when they are formed by a tran verse division of the germ ; and the defined 

 masses of yolk- spheres may be seen in all further stages ; they lie partly be- 

 tween the cords of vessels or endoblast-cells, which form the lower thick wall 

 of the yolk-sac, and partly at its free edge ; it has not yet been definitely shown 

 that they contain cell-nuclei. In the later stages of development free cells 

 may be distinctly seen within the yolk-sac; these are sometimes very 

 numerous; the cells are small, aud have a distinct nucleus; they are 

 irregularly scattered in the yolk-sac, and look as though they were 

 lymphoid cells. It is possible that some of the parablast-cells take a part 

 in the formation of the endoblast, but this point cannot be yet definitely 

 settled ; there is no reason to suppose that the cells arise or multiply 

 by free-cell formation. The author discusses in detail the supposition 

 of Kollmann that the germinal ridge (marginal ridge, KoUmann) is the 

 seat of origin of the blood ; and he comes to the conclusion that there is no 

 reason for accepting this hypothesis, or that a zone separated off from the 

 mesoblast gives rise to the blood-vessels. What KoUiker has shown to be 

 true of Birds and Mammals seems to hold also for Eeptiles. 



Maturation and Fertilization of Amphibian Ova.t — Dr. 0. Schultze 

 has been led by his studies on the ova of Amphibians to some general 

 results; he finds, as do those who have investigated the ova of other 

 classes of animals, that the germinal vesicle shares the fate of all the parts 

 that do not form the directive corpuscles, and passes into the substance of 

 the egg-cell ; greater weight must henceforward be laid on the fact that 

 there is a complete intermixture of the female nuclear substance and the cell 

 substance before fertilization. The nuclear substance which is collected 

 around the germinal vesicle as it commences its retrograde metamorphosis is 

 sharply separated, in some cases even by a temporary membranous layer, 

 from admixture with the substances of the egg. After a time this separa- 

 tion ceases, and the two parts soon unite. A part of the chromatic substance 

 passes to the surface of the egg, and then by a double mitotic division gives 

 rise to polar bodies; in the unripe egg of the Amphibia the germinal 

 vesicle occupies a central position so long as its fluid substances are equally 

 grouped in the direction of all the rays ; yolk-gemmules, which are quite 

 distinct, soon collect, and increase in size ; the egg then becomes telo- 

 lecithal. Objection is taken to van Beneden's epithet of " pseudo " as applied 

 to the karyokinesis which obtains in the egg of Ascaris megalocephala ; and 

 the author concludes with enumerating the proved cases of the presence of 

 polar globules in vertebrates. 



Structure of Ovum of Dipnoi.| — Mr. F. E. Beddard has a further con- 

 tribution to our knowledge of the structure of the ovum in Protojpterus, and 

 some notes on the ovary of Ceratodus ; in the latter form the multicellular 



* ZeitEchr. f. Wiss. Zool., xlv. (1887) pp. 282-307 (1 pi.). 



t Ibid., pp. 177-226 (3 pis.). 



X Proc. Zool. Soc. Loud., 1S86 (1887) pp. 505-26 (3 pis.). 



