PROCESS OF DEVELOPMENT IN AVIAN OVA. 529 



be distinguished from each other. A description of the cells of 

 the upper and lower germ laminse has already been given. 

 Those forming the middle lamina are small round cells with 

 uncommonly delicate outlines and elongated sharply defined 

 nuclei. 



As regards the relation of the laminae amongst themselves 

 they may be regarded as perfectly distinct, with the exception 

 of the middle part of the blastoderm, where the separation of 

 the middle from the upper lamina cannot be effected. 



Peremeschko has examined the variously named large ele- 

 ments lying at the bottom of the cavity on the warmed stage. 

 They change their form at a temperature of from 32 to 34 C. 

 This alteration of form usually consists in a primary contrac- 

 tion and loss of transparency, their form becoming oval or 

 irregularly round ; they then begin again to increase in size, 

 and to become more transparent. This alternate contraction 

 and enlargement was several times repeated in one example. 

 The changes in form were observable both in incubated and in 

 non-incubated eggs, but took place with great slowness. 



The accuracy of Peremeschko 's observations have been 

 corroborated by Oellacher. The perfectly unmistakable ap- 

 pearances presented in similar specimens I have also myself 

 so frequently seen, that I can entertain no doubt of the sub- 

 stantial correctness of his statements. 



If we now compare the disposition of the cleavage elements 

 in the germ of Batrachia with that of Fowls, it appears that 

 in Batrachia the external germ lamina, or the sensorial lamina 

 of Remak, is divided into two layers, whilst in the germ of the 

 Fowl it is undivided. In both cases, however, it is developed 

 from a superficial layer of cells v which is earlier differentiated 

 than the deeper lamina ; from the latter two other laminae are 

 developed, namely, the inferior and middle laminae. Thus at a 

 Certain stage there exists an opposition common to both between 

 an upper layer of smaller and a lower layer of larger cells. 



In Batrachia, as in Fowls, the larger and more slowly dividing 

 cells experience a partial dislocation, passing into a cavity de- 

 veloping coincidently with the process of cleavage. In the 

 Batrachia the large germ cells are attached to the roof of the 

 cleavage cavity, whilst in Fowls the large coarsely granular 



M M 



