TIIK EMBRYOLOGY OF EUVANESSA ANTIOPA. 101 



rial In the absorbed protoplasm the cell plasma becomes darker and the 

 still unaltered nucleus becomes very distinct. On the outside of the pro- 

 toplasmic layer there was a layer of grayish material ; this now forms a 

 cap over each cell and extends down each side for a considerable distance. 

 When all these cells have reached the circumference of the egg the blasto- 

 derm may be supposed to be fully formed, though at no time do all the 

 cells that form it resemble each other ; some commence their further de- 

 velopment before the others reach their proper position ; the blastoderm is 

 complete about twenty -four hours after deposition. Besides the blastoderm 

 cells there are in the centre of the egg a large number of other cells, 

 mostly yolk cells ; they have no definite arrangement but are pretty 

 evenly distributed over the whole egg. 



The blastoderm cells on one side of the egg continue to divide so that 

 when the blastoderm is complete the cells on one side are much smaller in 

 diameter than on the other ; they have, however, increased in thickness 

 and so make a thicker and more compact layer ; this is the beginning of 

 the ventral plate. The cells which make up this structure are at the bot- 

 tom of the egg and extend halfway up one side (63 : 10) , The transition 

 between this area is quite abrupt. In the further development the ventral 

 plate sinks deeper into the yolk. This is accomplished within three days 

 after deposition. The first indication of the process is a slight infolding 

 of the upper end (63: 10) ; the blastoderm cells begin to grow over the 

 ventral plate from this point and extend down the sides ; the edge of the 

 ventral plate sinks down at the same time. During this process of in- 

 folding the whole ventral plate begins an upward movement and increases 

 somewhat in size ; when the infolding is complete, that is, when the out- 

 folded edges of the blastoderm cells have met and closed over the whole 

 ventral plate, the latter is about as long as the egg but so curved as to 

 reach only about three-fourths Avay to the top (63 : 11) . It Avill be seen that 

 the embryo has now two layers of cells outside of it, one extending all 

 around and the other only across the outside face of the ventral plate ; 

 between these two layers the yolk penetrates freely. Great confusion 

 exists as to the nomenclature of these membranes but I prefer to follow 

 Balfour in this matter and designate the inner as the amnion and the outer 

 as the serous membrane, though the reverse is perhaps the more com- 

 mon practice. From this history of their formation it is evident that 

 both layers and the ventral plate are modified blastoderm cells, and 

 that the membranes can in no sense be called moultings of the ventral 

 plate. 



We will leave the embryo at this point and conclude by giving an ac- 

 count of a few structures that have already made their appearance, but 

 which have not as yet been noticed by us. One of these is a peculiar con- 

 dition of certain of the blastoderm cells at the upper pole of the egg ; it 



