1876.] On a Mammalian Ovum in an early Stage, 399 



within its centre a sudden display of very minute but richly coloured 

 Newtonian rings, formed in this case by interior reflected light, although 

 transmitted light was then being employed. 



It is not absolutely necessary that a plate of glass with precisely 

 parallel sides be used. A wedge can be manipulated if a particular spot 

 be chosen and the wedge be most carefully adjusted to the same position 

 by means of the stop and ledge on the stage. Less difficult, however, 

 is glass formed into a plano-convex lens of long focus, the plane side 

 being placed downwards, and the same point, the summit if possible, being 

 always selected for observation : better still if a slice be cut off so as to 

 present a secure fixing of the lens in the same position. 



A variety of substances formed into plates, wedges, or lenses, with 

 little convexity, may thus be examined, as also fluids enclosed between 

 parallel plates. 



V. "Description of a Mammalian Ovum in an early condition 

 of Development/'' By Edward Albert Schafer, Assistant 

 Professor of Physiology in University College, London. Com- 

 municated by Dr. Sharpey, F.R.S. Received March 8, 1876. 



[Plate 10.] 



The opportunities which present themselves for the acquisition of 

 the ova of Mammalia during the early stages of development, and 

 especially ova of the period during which the formation of the blasto- 

 dermic layers is proceeding, are so rare that, although the subject has 

 been under special investigation by more than one observer, all the 

 stages of the process of formation have by no means as yet been 

 described ; and much remains to be discovered in connexion with this 

 phase of development alone. It seems on this account desirable to 

 publish observations bearing upon this question, although they are 

 limited to two or three or even to a single ovum, since it is by collecting 

 and comparing the results which have been arrived at by different ob- 

 servers that there will be the best chance of coming to a definite conclu- 

 sion upon a subject which involves the knowledge of every progressive 

 stage. 



I have been induced by these considerations to furnish a short de- 

 scription of an early developing ovum (of the cat) which came into my 

 hands now fully two years ago, but which I had continually deferred the 

 account of in the hope of obtaining specimens a little more in advance 

 with which to compare that which I already had. This expectation 

 having, however, hitherto been disappointed, I think it better to com- 

 municate the description without further delay, as it may at least serve 

 for comparison with the statements of other observers. 



In a cat which had been just killed, I noticed on opening the abdomen 



