934 SUMMARY OF CURRENT RESEARCHES RELATING TO 



greatest growth-energy in the germinal substance; this energy seems to 

 depend on the direction of the planes of segmentation. 



Six hours after impregnation the blastodisc is formed, and lies flattened 

 on the now completely spherical yolk ; its outer surface is bounded by a 

 layer of flattened cells. Seven hours and a quarter after impregnation 

 nuclei appear in the part of the intermediate layer which is visible around 

 the margin of the blastodisc ; these group themselves in almost concentric 

 rows round this margin, and the rows are arranged in such a way that an 

 interspace of the succeeding row corresponds to every nucleus. After 

 pointing out the views held by previous writers with regard to these bodies, 

 the author states that he himself has observed clear vesicular nuclei 

 appearing round the edge of the blastodisc, and has found that they were 

 derived from the nuclei of its marginal cells. Nuclear figures were never 

 observed, but the author distinctly saw these nuclei constricted off from 

 those of the marginal cells ; the newly-formed structures increase in size 

 rapidly, and soon become disposed in rows. As to the significance of the 

 periblast the author hesitates to form a judgment, but the view that we 

 have to do with a conversion into nutrient material does not recommend 

 itself to him. 



In his second chapter the author deals with the formation of the embryo, 

 and compares his results with those of other embryologists who have studied 

 the development of fishes ; in the third chapter the development of the 

 eyes and ears, of the central nervous system and notochord, of the intestinal 

 tract and other parts of the body is described. 



Polar Bodies and Theory of Heredity.* — Prof. A. Weismann publishes 

 a confirmation and resume of his previous conclusions which have been 

 embodied in various papers since 1881. He regards it as a firmly estab- 

 lished and fundamental fact that all animal eggs which demand fertilization 

 give off two polar bodies as preparation for embryonic development, while 

 parthenogenetic eggs never extrude more than one. This fact, he says, 

 dismisses any merely morphological explanation of the precedents. If it 

 had no physiological signification, parthenogenetic eggs could retain the 

 portion of nucleus separated by a second division, no better than those 

 which demand fertilization. 



Dr. Weismann's opinion is as follows : — The first polar body represents 

 the extriision, after maturity is reached, of the too active protoplasm of the 

 nucleus; the second is the extrusion of part of the germ -plasm itself, 

 through which the quantity of original protoplasm from the parent is 

 reduced by one-half. A similar reduction must take place in the male 

 germinal cell, but it is impossible as yet to show this definitely from the 

 observed histology of spermatogenesis. 



Parthenogenesis occurs when the whole of the germ-plasm from the 

 parent is retained within the egg-cell. Sexual reproduction demands that 

 half the germ-plasm be extruded from the egg, so that the remaining half 

 may again reach the required size by uniting with the sperm-nucleus. 



In both cases the beginning of development depends on the presence of 

 a certain, and indeed of the same quantity of germ-plasm. The fertilized 

 egg gains this by the addition of the sperm-nucleus, and the commencement 

 of embryonic change follows right on the heels of fertilization. The 

 parthenogenetic egg contains from the first the necessary amount of germ- 

 plasm, and it becomes active as soon as the extrusion of the single polar 

 body has freed the egg from the " oogenetic " nuclear plasm. 



* Weismann, A., ' Ueber die Kichtungskorper, und uber ihre Bedeutung fiir die 

 VercrbuDff,' 1887. 



