978 SUMMARY OF CURRENT RESEARCHES RELATING TO 



the segregation of the chorda dorsalis, and of the mesoblast plates, 

 together with the primitive hypoblast, form the cells of the gut, and 

 may be termed the endoblast. 



With reference to the formation of the mesoblast, it is to be noticed 

 that the secondary hypoblast from which it arises is composed of cells 

 partly hypoblastic and partly epiblastic. 



Position of the Yolk-Blastopore as determined by the Size of 

 the Vitellus.* — Mr. J. A. Ryder, who has already shown that in the 

 Teleostei the portion of the point of closure of the blastoderm in 

 relation to the original j^osition of the germinal disc is largely 

 determined by the size of the vitellus, now considers other vertebrates. 

 In two large yolked types — the Elasmobranchs and the Sauroids — the 

 embryo is displaced in position in reference to the margin of the 

 blastoderm. In them the embryo is partially folded off, whereas in 

 other fishes and in Amphibians the embryonic axis extends back to the 

 point where the yolk-blastopore closes ; the difference is due to the 

 great difference in the bulk of the yolk. 



The germinal disc of Sauroids is relatively much larger than that 

 of Teleosteans, so that proportionally it probably does not spread 

 over a much larger vitelline surface in the one than in the other case ; 

 the two forms of closure may be distinguished as teleporous and 

 ateleporous. " The ova of the two extremes of the vertebrate series — 

 Branchiostoma and Mammalia — are yolkless, except those of the 

 Monotremata, which are probably ateleporous " — the band of tissue 

 from the vitelline end of the umbilical stalk to the edge of the 

 blastoderm-rim in Elasmobranchs, and the primitive streak in Sauroids 

 and mammals are probably homologous structures. The Sauroids 

 present other differences in the fact that the germinal wall is not 

 carried quite to the border of the blastoderm all round, as in the 

 Ichthyopsida, and this again appears to be due to the quantity of 

 yolk. 



The blastopore observed by Van Beneden in mammalian ova is 

 not a true blastopore ; the degenerate condition of the yolk of these 

 eggs may probably be due to the development of the so-called uterine 

 milk, by which the egg is nourished before the foetal vessels are 

 developed. 



On the other hand, viviparity has not affected the development 

 of the yolk in Teleosteans, and it seems to be quite conceivable that 

 the mammalian vitellus, like the ambulatory or prehensile organs of 

 parasitic organisms, may have been atrophied in consequence of the 

 perfectly paiasitic connection which obtains temporarily between the 

 maternal organism and the embryo. 



Development of Viviparous Osseous Fishes.f — Mr. J. A. Ryder 



gives a lengthy summary of our knowledge respecting the best known 

 of the truly viviparous osseous fishes characterized by an intrafollicular 

 or intra-ovarian development. New observations are recorded on the 

 changes undergone by the embryos of the Embiotocoids during gesta- 



* Amer. Naturalist, xix. (1885) pp. 411-5. 



t Proc. U.S. Nat. Museiim, viii. (1885) pp. 128-55 (4 pis.). 



