PROCESS OF DEVELOPMENT IN AVIAN OVA. 527 



Peremeschko, however, states that the large granular cells 

 on the floor of the germ cavity augment considerably in num- 

 ber in the course of the first hours of incubation. And, since 

 with this increase in number there is no corresponding 

 diminution in size, it becomes highly probable that the cells 

 projecting downwards from the inferior lamina fall to the 

 bottom of the cavity. This is the more likely, as it is obvious 

 from the appearance above described, that a part of the 

 cleavage elements lying in the lower segment of the germ 

 remain lying upon the floor of the cavity when the germ rises 

 in order to form this cavity. 



The lower of the two primary laminse is, in my opinion, 

 therefore not identical with that which Eemak has described 

 under the same designation. According to Remak, the middle 

 and what is subsequently the inferior layer separate from this 

 lower lamina. This, however, is not really an accurate state- 

 ment of the case. The originally inferior lamina consists, at 

 least before the middle one is formed over the germ cavity, of 

 a layer of flat cells, and it preserves this structure long after 

 the middle lamina is formed. The much thicker middle layer 

 cannot arise by dilamination from this layer of flat cells. 



Peremeschko has met with the first traces of the middle 

 lamina at about the seventeenth hour of incubation. The 

 statement of the exact number of hours has only an approxima- 

 tive value, since both the brooding temperature* and the con- 

 dition of the ovum at the commencement of incubation must 

 be taken into consideration. With this precautionary observa- 

 tion I shall here follow the statements of Peremeschko. 



At about the seventeenth hour of incubation, then, coarsely 

 granular elements occur here and there between the upper and 

 lower laminse, which, both in regard to their size and contents, 

 differ essentially from the cells of either of these laminae, but 

 agree perfectly with those that lie on the floor of the cavity ; 

 soon after this the rudiment of the central part of the middle 

 lamina appears. In some preparations it may be observed that 



* I use a water bath for the purpose of incubation, maintained at a 

 tolerably uniform temperature of 39 C. (102 Fahr.) by a self -regulating 

 gas flame. 



