20 Gr. 0. Sårs. 



gastrula-mouili. It is at first encircled by a rather regular 

 rosette of somewhat cuneiform ectodermal cells, the number 

 of which is generally 8, though in some instances only 7 

 such cells are counted, as indicated on fig. 11. The ovum 

 in this stage still exhibits a perfectly globular form (see 

 figs. 10, 11), and the cleavage process is far from being 

 finished, but continues rapidly also in the succeeding stages. 



IV. 

 Gastrulation of the ovum. 



This process is commenced by the formation of the 

 above-described small, rounded depression at the one pole 

 of the egg. It is from this place that the gastral hollow 

 will subsequently be formed, as also the 2 inner cellular 

 layers, the mesoderm and entoderm; but in the stage here 

 treated of, strictly speaking, only a single distinctly defined 

 cellular layer is as yet formed, viz., that of the ectoderm, 

 the entodermal cells still constituting a compact mass filling 

 up the central part of the ovum. On a closer examination, 

 a number of small cells may be detected lying just inside 

 the edges of the blastoporus, being, in stained eggs, distin- 

 guished by a much deeper colour than the others. These 

 cells, which are apparently cut off from the inner ends of 

 the adjacent ectodermal cells, soon begin to divide, and 

 grow inwards between the ectodermal cells and the entodermal 

 cells, thus undoubtedly representing the first commencement of 

 the mesoderm (cf. fig. 14). The central entodermal mass at 

 the same time gradually assumes the character of a defined layer 

 by the invagination of the gastral hollow. In more advanced 

 stages (fig. 12) the gastral pole becomes more flattened, so 



