CHAPTER III. 



FOKMATIOX OF THE LaYEP.S. 



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 (PL 

 III. fig. 1). 



This condition of the segmentation cavity, though already^ 

 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 encounterino- 

 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 bJastoderm. Dr Schultz" 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^. If this is the true view of its nature the fact should be 

 noted that at first its floor is formed by the lower layer cells 

 and not by the yolk, and that its roof is constituted by both the 



^ Qy. Journal of Microsc. Science, Oct. 1874. 



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



3 Professor Bambeke {Poissons Osseux, Mem. Acad. BeJgique 1875) describes 

 a cavity in the blastoderm of Leuciscus mtilus, which he regards as the true 

 segroentation cavity, but not as identical with the segmentation cavity of 

 Osseous Fishes, usually so called. Its relations are the same as those of my 

 segmentation cavity at this stage. This paper came into my hands at too late 

 a period for me to be able to do more than refer to it in this place. 



B. 3 



