100 THE BUTTERFLIES OF NEW ENGLAND. 



very indistinct at first ; this appearance does not begin at one point and 

 spread over the spherule, but appears simultaneously over the whole 

 spherule. These granules, which are from the first coarse, become more 

 and more distinct and highly refractile, and finally the outline of the 

 spherule is no longer discernible. The bodies produced are not fat, 

 although they resemble it very much in optical qualities. They may be 

 looked upon as a form of albumen most readily converted into cell food, 

 while the yolk-like fat is a condition of the food material suited for stor- 

 age. Yolk degeneration goes on with great rapidity, until about one- 

 third of the mass of the egg has assumed this granular condition ; this is 

 at the time the newly formed blastoderm cells begin their migration 

 towards the circumference, when a pause ensues. The degenerated yolk 

 occupies the centre of the egg, and the blastoderm cells readily push their 

 way through this material and pass into the part containing the undifferen- 

 tiated spherules beyond. After the blastoderm cells have passed this line 

 the yolk cells follow and carry with them some of the degenerated yolk, 

 and finally produce the yolk masses as described below. The process of 

 defeneration now takes a new start from the nuclei of the yolk cells and 

 spreads gradually outward from each, until the whole mass has undergone 

 this change. One of the most striking stages (63: 5) is when the whole 

 mass has degenerated, except a single row of spherules around each cell. 

 The degeneration of the yolk is not complete till quite late in the develop- 

 ment of the insect. Some recent authors have described bodies in the 

 yolk spherules of insects, which they call bacterioids, attributing to them 

 certain bacteria-like action. Their method of appearance, hoAvever, seems 

 to exclude the possibility of their being anything like organisms growing 

 in the yolk. 



The earliest stage known in the development of the egg is when there 

 are about twenty cells present. These are about uniform in size and all 

 at quite a distance from each other, for at this stage as soon as a cell 

 divides the resultant cells separate. This is facilitated by the degenerated 

 condition of the yolk spherules in this region ; the cells are ameboid in 

 shape and the nucleus very indistinct, but of considerable size ; after di- 

 vidino- several times the cells arrange themselves in line and commence a 

 migration towards the circumference. In going through the degenerated 

 yolk they sometimes leave trailing out behind them a long process (63 : 3) 

 of protoplasm ; on reaching the edge of this region they pause, gather 

 themselves together, and plunge into the mass of undifferentiated yolk. 

 While in transit, the cells divide so as to keep about the same distance 

 apart ; they do not all reach the edge at the same time, but those on one 

 side take their station long before the others (63 : 4) . On reaching 

 the protoplasmic layer the cells at once appropriate that immediately be- 

 fore them and so increase rapidly in size. Owing to the granular mate- 



