182 QUARTERLY CHRONICLE OF MICROSCOPICAL SCIENCE. 



in a chain, are attached to the peripheral parts of the germinal 

 disk (the formative elements of the middle layer, Peremeschko). 

 Oellaclier has shown that they are to be regarded as elements 

 produced by segmentation as derivatives of the germinal 

 disk, Avhich during the formation of the segmental cavity 

 remain behind on its floor. With respect to the changes in the 

 germinal disk which take place when incubation commences, 

 and more particularly with respect to the participation of 

 the last-named elements in the formation of the middle 

 germinal layer, Klein confirms essentially the statements 

 of Peremeschko. That is to say, these cells divide, and the 

 small cells derived from them, which are easily recognised by 

 their large nuclei, penetrate (first of all in the centre of the 

 area pellucida) into the space between upper and lower 

 germinal layer. Klein decisively rejects the theory of His, 

 that the middle germinal layer which arises in the first day 

 of incubation between the upjier and the lower is formed by 

 the proliferation and horizontal extension of the sub-germinal 

 processes. On the contrary, the derivatives of the formative 

 elements of the middle layer migrate from the outside of the 

 disk between the two germinal layers towards its centre. 



When at the end of the first day of incubation a distinct 

 and continuous middle layer is thus completely formed 

 between the upper and lower layer in the manner above 

 described, contemporaneously with the production of the 

 primitive groove, then next begins a considerable enlarge- 

 ment and tbickening of the middle layer, proportionate to 

 the enlargement outwards of the upper layer. During the 

 whole period of growth and till its actual completion the 

 middle layer forms no anatomical connection with the upper 

 layer or with the white yolk of the germinal wall. Much 

 stress is laid upon this point, as op])Osed to the theory of His, 

 about the reception of parablastic elements into the germinal 

 disk from the white yolk. 



After these general considerations, Klein proceeds to 

 describe the histological facts which he has observed in the 

 development of blood-vessels and formation of blood-corpuscles 

 as seen in fresh objects by a surface view. All stages of 

 this development may be traced in the area pellucida of 

 almost any germinal disk which is fresh and carefully put 

 under the microscope in the first half of the second day of 

 incubation. 



The three folloAving series of changes are distinguished : 



(a) Some of the cells of the deeper layer of the germinal 

 disk become hollow by the formation in them of a vacuole, 

 and by enlargement of the vacuole are transformed into 



