SOME POINTS IN EARLY DEVELOPMENT OF HEn's EGG. 161 



der Phys.,' 1853, Bd. iv) also describes segmentation in the 

 unfertilised ova of frogs. 



Hensen (* Centralblatt/ 1869, No. 26) noticed segmenta- 

 tion in the unfertilised ova of rabbits. 



Waldeyer, " Die Epithelialen Eierstocksgeschwiilste," 

 ' Archiv fiir Gyngekologie,' 1870, ii Heft, suggested that the 

 dermoid cysts of the ovary may have a parthenogenetic 

 origin from the ova therein. 



I now proceed to describe the irregular segmentation that 

 I have observed myself. In seven eggs that were incubated 

 for different periods, varying from eight to fourteen hours, 

 it was noticed that segmentation had apparently recently 

 begun or had proceeded very slowly. In all these cases 

 segmentation was not nearly as far advanced as in normal 

 unincubated eggs. The finely granular blastoderm in sec- 

 tions of some of these eggs appeared to be divided up into a 

 comparatively small number of large irregular masses, some 

 of which appeared about to divide into smaller masses. In 

 other cases the blastoderm was seen to be divided into two 

 parts, separated from each other by an interval as in figs. 1 

 and 2. In these latter cases segmentation was more ad- 

 vanced than in the preceding cases. In each instance the 

 blastoderm rested on a finely granular bed or layer, which 

 separated the blastoderm from the white yolk. In places 

 this finely granular layer was sharply defined towards the 

 germ, in other places it seemed to pass insensibly into it. 

 But in all cases this layer gradually shaded off more or less 

 into the subjacent white yolk. This finely granular or 

 " subgerminal layer," as we shall call it, sometimes formed 

 a continuous floor beneath the blastoderm from one end to 

 the other; at other times it was massed in wedge-shaped 

 blocks at one or either end of the blastoderm, and was con- 

 tinued as a fine strip towards the centre beneath the blasto- 

 derm. These wedge-shaped peripheral thickenings of the 

 subgerminal layer were very clearly marked in some cases, 

 as will be seen on referring to the figures 3, 4, 5, and 6. 



In a few sections where these peripheral thickenings were 

 most marked the subgerminal layer was seen to be more 

 sharply defined at these points towards the subjacent yolk, a 

 fine cleft separating them in certain cases. The lateral ex- 

 tension of the subgerminal layer over the white yolk at the 

 periphery of the blastoderm varied considerably in extent, 

 and here as throughout its extent this finely granular layer 

 was more or less beset with vacuoles. 



In a few instances these vacuoles contained a large cell- 

 like mass exactly resembling the segmentation masses of 



