230 MR GEORGE BROOK ON THE 



cell outline is polygonal, ■whereas where there are fewer nuclei the cell contour 

 is more rounded. So far as I am aware, this stage represents the earliest one 

 at which cells from the parablast have been described as taking part in the 

 formation of the embryo in other Teleostean types. It corresponds to that of 

 fig. 2 in my account of the pseudo-invagination in Trachinus (6). The 

 extension of the morula over the yolk, the formation of the segmentation 

 cavity, and the differentiation of the germinal layers has not yet commenced, 

 though these changes immediately follow on this stage. So far as my investi- 

 gations go, I am not aware of any case exactly comparable with this, though 

 further investigations may probably reveal such. In all the Teleostean types 

 with which I am acquainted, with the exception of the herring, the primary 

 segmentation process results in the formation of a morula corresponding to 

 that of fig. 18, and it is only in stages immediately following this that the 

 elements resulting from secondary segmentation take part in the further 

 differentiation of the blastoderm. In other words, in most Teleostean ova, the 

 morula consists solely of archiblastic elements, and it is only after the formation 

 of the segmentation cavity, and the commencement of invagination, that the 

 parablastic elements come to be utilised. In the herring, as we have seen, at 

 least two distinct batches of parablast cells are budded off, and unite with those 

 of the archiblast before any trace of differentiation of the morula is to be found. 

 The final morula in this case contains parablastic as well as archiblastic 

 elements. The difference is important, and as I hope to show later, is con- 

 nected with the early elaboration of the parablast, and probably also with the 

 absence of a vitelline circulation in this type. 



Shortly after the stage represented in fig. 18 the morula begins to spread 

 out over the yolk, and the extension is accompanied by a thinning out of the 

 central portion, which has hitherto been thickest. In this way a segmentation 

 cavity is formed, which, however, never reaches so important a development 

 in the herring as in some other Teleostean types. The parablast forms the 

 floor of the segmentation cavity, while the cells of the morula form its roof and 

 lateral boundaries. Fig. 21 (PI. XV.) represents a longitudinal section in the axis 

 of the embryo 68 hours after fertilisation. The roof of the segmentation cavity 

 is formed of several rows of cells, the most external constituting the epidermal 

 layer of the epiblast. Towards the periphery of the blastoderm there is a 

 thickening forming the commencement of the blastodermic rim. There is no 

 invagination of the epidermal layer of the epiblast, but the thickening may 

 possibly be in part produced by an invagination of the nervous layer. I am, 

 however, inclined to believe that the thickening, in so far as it does not result 

 from an addition or segregation of cells from the parablast, is due to mechanical 

 agencies. As the cells forming the roof of the segmentation cavity press 

 towards the periphery, so as to aid the blastoderm in its extension over the 



