136 RICHARD ASSHETON. 



in one specimen, in which the albumen layer and zona radiata 

 had been removed (thereby rendering it very likely that a tear 

 may have occurred), and he was quite unable to find it in other 

 specimens. If such an aperture really existed, it would cer- 

 tainly be perceptible in surface views of specimens stained with 

 silver nitrate. Van Benedeu makes no mention of such an 

 aperture. I have hunted carefully but can fiud none, 

 either in surface views of specimens treated with silver 

 nitrate and other reagents, or in series of sections; in 

 fact, I should be greatly surprised to find such an aperture 

 at the time given by Keibel for its appearance. It 

 seems to me quite useless to look for it, since it is sup- 

 posed to represent a turning inwards, such as can be seen in 

 Amphibian ova, for the Mammalian embryo at this age is 

 utterly unlike the Amphibian embryo at the corresponding 

 age, and therefore the conditions which lead to the apparent 

 turning in as seen in Amphibia are hardly likely to be existent 

 in the case of the Mammalian embryo. 



It may be well to point out that this is not the view of 

 so great an authority as Dr. Oscar Hertwig, who says, 

 p. 90 of his text-book, when speaking of this spot, *'Von 

 dieser Stelle aus, nehme ich an, hat sicli schon auf einen 

 noch friiheren Stadium das untere Keimblatt durch Um- 

 schlag eines kleinen Bezirks der einblatterigen Keimblase 

 entwickelt." 



The changes that occur during the fifth day affect both inner 

 mass and outer layer. 



The outer layer consists throughout the fifth day of flattened 

 lenticular cells, whose boundaries are easily recognisable in 

 surface views, and I have nothing to add to van Beneden's 

 careful and accurate description of them. They are of very 

 various shapes and sizes, the lines which mark their boundaries 

 may be at any angle to each other. The boundaries may 

 number six, five, or four, the most usual form being the hexa- 

 gonal. 



As the vesicle enlarges the cells become rather smaller, and 

 become much less regular in outline, so that by the seventh 



