102 THE BUTTERFLIES OF NEW ENGLAND. 



IS quite common if not universal in the eggs of insects, as one can per- 

 ceive by examining the figures of young insect embryos by various 

 authors. No one, as far as I can make out, has expressed any opinion as 

 to their significance or function, nor can I. Very early in the formation 

 of tlie blastoderm, certain of the cells in the upper end fuse together and 

 form a syncytium ; their nuclei are large, and the whole mass is larger 

 than an equal number of blastoderm cells, even of those in the ventral 

 plate in the time of their maximum size ; beneath them the yolk assumes 

 a peculiar condition possessing bubble-like cavities. 



The yolk cells constitute another of those structures which we may call 

 temporary ; they perform the very necessary function of preparing the 

 food material out of the yolk. It has not been proven that any of them 

 take any part in the formation of the tissues of the insect, and many of 

 them certainly do not ; at first they resemble very much the cells that go 

 to make up the blastoderm, but may be distinguished by their affinity to 

 the yolk, or rather by the fact that they absorb the yolk spherules and 

 granules bodily ; shortly after blastoderm formation they complete their 

 absorption of the yolk and are called yolk masses, but may with strict 

 propriety be still called yolk cells. The method of yolk degeneration 

 that these cells set up has been already described. There are other cells 

 in the mass of the egg which do not take part in the degeneration of the 

 yolk ; they appear as indiiferent cells of the earlier stages, and probably 

 give rise to the endoderm. Others find their way to the ventral plate 

 and lie close against it ; these appear to give rise to the mesoderm, though 

 this is not the view generally received as to the origin of this germ layer. 



About the time of the completion of the blastoderm the already diflPer- 

 entiated ventral plate infolds at a point on the median line about two-thirds 

 from the upper end and forms a very narrow pocket. The cells composing 

 it look like the rest of the cells of the ventral plate at this time ; they are 

 almost round and have a lining on one side made of the gray matter which 

 originally bordered the whole egg but which became a part of the blasto- 

 derm cells. The pocket remains open but a short time, but there is a long 

 depression at the upper end of the bunch of cells : the mass of cells are 

 soon cut ofll^ from the ventral plate and are free in the body cavity, but 

 remain in contact with the ventral plate at the point where they were pro- 

 duced. Later stages show that these cells produce the generative organs ; 

 the generative organs thus appear to be produced by an infolding of the 

 ectoderm, or possibly of the l)lastoderm before the ectoderm is produced, 

 but from a portion Avhich is later to become ectoderm. The general idea 

 has been that the generative organs in insects are produced from the 

 mesoderm, although Metschnikow as early as 186fi showed for certain 

 insects a difl^erent origin. 



Tiiis brings us face to face with an unsettled question of fundamental 



