1 6 DEAN. [Vol. XI. 



disk in a vertical plane ; it results in a trench-like groove 

 showing the yolk mass below separating the blastomeres ; at 

 either end it bows outward, rounding the corners of the germ 

 disc, and extends no further down the side of the eggs in any 

 example the writer has examined ; the margins of the furrow 

 are boldly marked, highest at the animal pole of the ^^%. 

 Sections at this stage indicate that the germ disk is more 

 clearly to be distinguished from the underlying yolk, the 

 nuclei occupying a relatively higher plane than in the preced- 

 ing figure ; the furrow is seen (PI. II, Fig. 22), to leave below 

 it undivided a well-marked layer of the germinal protoplasm. 

 The first furrow has been observed ^ to divide the germ disk 

 into segments of unequal size, an abnormality of cleavage well- 

 known in Teleosts,^ which was found to influence in no way 

 subsequent development. 



Second Cleavage (PI. I, Fig. 3), in all cases examined, occurs 

 in a vertical plane approximately at right angles to the first. 

 It is expressed in the germ disk only, and like the former 

 furrow could not be traced in the yolk region of the egg. The 

 polar corners of the blastomeres are the most prominent, 

 sharply cut, and but slightly rounded. The nuclei remain in 

 the horizontal plane of those of Fig. 2, and no change is seen 

 to occur in the layer of protoplasm underlying the furrows. 

 During cell division the nuclear changes are not prominent : 

 the spindles are seen with difficulty, the chromosomes are 

 small and obscure, and in the resting stage the outline of the 

 nucleus can hardly be determined. In general the position of 

 the dividing nuclei with respect to the blastomeres is similar to 

 that of amphibian : shortly after division the nuclei are seen to 

 be separated by a thick transparent disk of protoplasm, a seg- 

 mentation plane which is later expressed in the surface furrow 

 (PI. II, Fig. 23). 



Third Cleavage (PI. I, Fig. 4) is also in a vertical plane : in 

 general its direction is parallel to the first furrow ; ^ often, 



1 By Prof. E. B. Wilson. 



2 E. g. Ryder in the Cod and H. V. Wilson in Serranus. 



2 This was accurately determined by E. B. Wilson in his experiments upon eggs 

 attached to glass plates whose various cleavages were recorded. 



