MUSEUM OF COMPARATR^E ZOOLOGY. 67 



II. — The Embryo. 



For convenience in describing the development of the embryo, the 

 following periods may be recognized : — 



(1.) The preblastodermic period, in which are embraced the changes in the 

 mature egg up to and including the formation of the blastoderm. 



(2.) The period from the completion of the blastoderm to the formation of the 

 rudimentary appendages, embracing, (a) the invagination, (6) the stage 

 of the primitive cumulus, (c) the formation of the ventral plate, and (d) 

 the division into protozonites. 



(3.) The period from the appearance of the appendages to the reversion of the 

 embryo. 



(4.) The period of reversion. 



(5.) The period from the reversion to the hatching of the embryo. 



1. Preblastodermic period. — The superficial and internal changes, al- 

 though going on simultaneously, may be more easily described if consid- 

 ered separately. The surface changes can be watched on the living egg, 

 and have been already thoroughly studied; but it is impossible to under- 

 stand fully these changes without that knowledge of the internal pheno- 

 mena which is to be obtained only by the aid of sections. The lack of 

 this method of study has led several previous observers into errors of 

 interpretation. 



My earliest observations on the eggs of Agelena noevia were made a 

 few (probably not more than three or four) hours after their deposit. At 

 this time the polarity of the egg is very apparent ; one hemisphere is 

 characterized by small yolk corpuscles packed closely together, though 

 not joined in masses, and the other by agglomerations of larger yolk 

 corpuscles. The irregular masses thus formed are separated by spaces in 

 which are found smaller isolated corpuscles like those which distinguish 

 the opposite hemisphere. 



Balbiani (73) was the first to give an adequate account of the surface 

 changes which occur during this period, in which the peripheral layer of 

 protoplasm is principally concerned. We shall see by following the his- 

 tory of this protoplasmic layer, that it is the equivalent of the blastema 

 observed in the eggs of many insects (Diptera, Phryganids, etc.), and 

 Crustacea (crab, etc.), and I shall so designate it hereafter. 



Ludwig (76) and Barrois (78) have both called in question the accu- 

 racy of Balbiani's observations as to the peculiarities of the blastema. 

 My own observations are more in agreement with those of Balbiani, 

 which they serve in a measure to confirm. 



