ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 563 



In frout of the medullary groove lies a completely closed mass of 

 mesoblast ; tlie interamniotic pore, described by E. van Beneden and Julin, 

 was not observed in young germinal areas. 



The anterior amniotic fold in the cat, dog, fox, and mole is not covered 

 by mesoderm, but consists wholly of ectoderm and entoderm. It follows 

 from this that there is found a proamnion not only in Eodents, Bats, and 

 Marsuj)ials, but also in Carnivora and Insectivora, from which it may be 

 concluded that it is a structure common to the Mammalia. The significance 

 attached to it by Van Beneden the author cannot share. 



The Wolffian duct does not arise as a solid cord of cells, but, as the 

 author observed in the duck, as a diverticulum of the ccelom ; that the 

 ectoderm takes part in the formation of the Wolffian duct was not estab- 

 lished. 



As respects the formation of the maternal placenta, the author fully 

 confirms the statements of Bischoff, that the villi of the chorion grow into 

 the uterine glands, destroying the latter. 



Embryolog'y of Monotremata and Marsupialia.* — Mr. W. H. Caldwell 

 has published an abstract of the first part of his paper on the development 

 of Monotremata and Marsuj)ialia. In very young ova of the former there is 

 a fine membrane between the single row of follicular cells and the substance 

 of the ovum ; this vitelline membrane at first increases in thickness with 

 the growth of the ovum, and numerous fine protoplasmic processes pass 

 through it and connect the protoplasm of the follicular cells with that of 

 the ovum ; these serve for a time to conduct food granules. This " yolk- 

 forming period " is succeeded by an " absorption of fluid period," during 

 which the ovum absorbs large quantities of fluid, and increases in size ; the 

 third period is that of the formation of the chorion. All these periods are 

 gone through while the egg is still in the follicle. In the passage of the 

 egg along the Fallopian tube the vitelline membrane again increases in 

 thickness, and the chorion absorbs fluid and becomes the albumen layer ; 

 outside this now appears the shell or shell-membrane, which is tough and 

 parchment-like, without calcic salts in Echidna, but apparently with them 

 in Orniihorhynchus. The deposition of the shell has not yet been observed 

 to be due to the activity of any special glands, but the author says the 

 shell-membrane does not increase at the expense of the chorion or albumen 

 layer. In Marsupials the yolk-forming period is not marked off from the 

 absorption of fluid period ; in an ovum of Phascolarctos there was a thin 

 transparent shell-membrane. 



The ova are telolecithal, and go through a partial segmentation ; though 

 the ova of Placentalia segment completely, the resulting blastodermic 

 vesicle is identical with that of Monotremes and Marsupials. A primitive 

 streak region is formed, in Monotremes, in front of the posterior lip to the 

 blastopore, and long before the epiblast has enclosed the yolk. In 

 Marsupials the epiblastic growth encloses the hypoblast at a very early 

 stage, except over a narrow slit in front of the posterior lip of the blastopore ; 

 the primitive streak is not conspicuous at an early age, because of the large 

 size of the cells. Balfour's objection to the comparison of the blastopore 

 of the rabbit with that of the frog is explained by the presence of a posterior 

 lip to the blastopore in Marsupials ; the author postulates the existence of 

 a similar structure in the rabbit, and regards its blastopore as corresjjonding 

 to the whole area marked out by the growing epiblast and the posterior lip 

 of the blastopore before the closing of the primitive streak region. 



* Proc. Roy. Soc. Lond., xlii. (1887) pp. 177-80. 



