CHAPTER III. 

 FORMATION OF THE LAYERS. 



IN the last chapter the blastoderm was left as a solid lens- 

 shaped mass of cells, thicker at one end than at the other, 

 its uppermost row of cells forming a distinct layer. There 

 very soon appears in it a cavity, the well-known segmenta- 

 tion cavity, or cavity of von Baer, which arises as a small space 

 in the midst of the blastoderm, near its non-embryonic end 

 (PI. 7, ng. i). 



This condition of the segmentation cavity, though already 1 

 described, has nevertheless been met with in one case only. 

 The circumstance of my having so rarely met with this con- 

 dition is the more striking because I have cut sections of a 

 considerable number of blastoderms in the hope of encountering 

 specimens similar to the one figured, and it can only be explained 

 on one of the two following hypotheses. Either the stage is 

 very transitory, and has therefore escaped my notice except 

 in the one instance ; or else the cavity present in this instance 

 is not the true segmentation cavity, but merely some abnormal 

 structure. That this latter explanation is a possible one, appears 

 from the fact that such cavities do at times occur in other parts 

 of the"blastoderm. Dr Schultz 2 does not mention having found 

 any stage of this kind. 



The position of the cavity in question, and its general ap- 

 pearance, incline me to the view that it is the segmentation 

 cavity 3 . If this is the true view of its nature the fact should be 



1 Qy- Journal of Microsc. Science, Oct. 1874. [This Edition, No. V.] 



2 Centr.f.Med. IViss. No. 38, 1875. 



3 Professor Bambeke (" Poissons Osseux," Mhn. Acad. Belgique 1875) describes a 

 cavity in the blastoderm of Leuciscus rutilus, which he regards as the true seg- 

 mentation cavity, but not as identical with the segmentation cavity of Osseous Fishes, 



