400 Mr. E. A. S chafer on a Mammalian Ovum [Mar. 23, 



five scarcely perceptible swellings in the cornua uteri ; and perceiving a 

 corresponding number of corpora lutea upon the ovaries, judged that 

 each swelling must contain an ovum, Eemoving the uterus and placing 

 it in a weak solution of bichromate of potash, I proceeded carefully to 

 slit open the cornua under the fluid with fine scissors. As each one of 

 the above-mentioned dilatations was reached, a minute, beautifully clear, 

 vesicular body floated out into the surrounding liquid ; there was no sign 

 of any adhesion to the uterine wall. 



The vesicles were as nearly as possible similar in size and appearance, 

 and a description of any one of them will serve for all. Their shape was 

 oval, the long diameter measuring about \ inch, the short about T 1 ^- inch, 

 and the outline being perfectly smooth and even. Under a low power of 

 the microscope the vesicle was distinctly seen to be bounded by a primitive 

 chorion or thinned-out zona pellucida. No trace of villi or projections 

 of any sort could be detected on its surface. Besides this envelope 

 the wall of the vesicle was composed of what appeared a simple layer of 

 flattened polygonal cells, closely lining the zona. But under a somewhat 

 higher power a stratum of more deeply lying cells could, in some parts 

 at least, be detected by focusing ; moreover a shadow at one place 

 midway between the poles of the oval appeared to point to the possibility 

 of the existence of a slight thickening at this part, although a well- 

 defined shaded area was, in no sense of the word, visible. 



I was led to imagine that the " shadow " in question, or rather the 

 thickening to which it was probably due, would be caused by the first 

 beginnings of a mesoblast at this situation. But nothing more could 

 be made out in the fresh condition, and the little vesicles (at least two of 

 them, for the others were of less value for the purpose of sections, owing 

 to the blastoderm having shrunk away at various places from the zona, 

 and presenting a crumpled distorted aspect) were accordingly hardened 

 in the usual way in very dilute chromic acid, stained with logwood and 

 with carmine respectively, imbedded by the gum-process*, with the object 

 of filling the cavity and thus supporting the enclosing parts and preserv- 

 ing them in their natural positions ; and finally sections were made across 

 the long axis of the oval, and were mounted in glycerine and examined. 



A glance at the sections is sufficient to show that the blastodermic 

 vesicle is in the bilaminar condition. There exist within the zona 

 pellucida, or primitive chorion, two distinct layers, the section of each 

 forming a complete circle, the whole structure, therefore, included by the 

 zona being formed of two separate vesicles one within the other (Plate 

 10. fig. 1). The outer of these is of course the epiblast, the inner 

 doubtless representing the hypoblast : we may speak, then, of an epiblastic 

 and a hypoblastic vesicle. In none of the sections was there any trace 



* A bad method for embryos ; but I was at the time ignorant of Kleinenberg's ex- 

 cellent plan for effecting the same object. See Eorster and Balfour, 'Elements of Em- 

 bryology,' p. 249. 



