1 8 DEAN. [Vol. XI. 



their subsequent derivatives, as bb, bbb, or cc, ccc. The writer 

 believes, accordingly, that the plan of the fifth cleavage is 

 carried out in the marginal cells' budding off their polar ends, 

 giving rise to a circle of twelve cells ; the four cells of the 

 animal pole dividing obliquely in a meridional plane. This 

 mode of fifth cleavage corresponds clearly with that of the 

 ideal type in Teleost segmentation (H. V. Wilson). In many 

 examples of this stage the present writer notes that the 

 four cells of the animal pole undergo horizontal cleavage, 

 so that in surface view but twenty-eight blastomeres may be 

 counted. 



From the fifth cleavage onward the division of cells could 

 not be satisfactorily followed, and the outward appearance of 

 similar stages presents so many variations that they seem 

 valueless to record. In the sixth cleavage (PI. I, Fig. 8), the 

 only cell divisions that were noted as generally constant were 

 those of the marginal cells : these undergo meridional cleavage, 

 similar to the former one, its furrows extending no further than 

 the margin of the cell-cap. ^ It is in fact in this stage that the 

 cell-cap is largest in outward size. Horizontal cleavage has 

 taken place irregularly, and has caused the lower layer of the 

 germ protoplasm to be broken up into irregular cells whose 

 upper and marginal limits are readily traced, but whose cyto- 

 plasm is seen clearly in many cases confluent with the yolk. 

 This layer of yolk cells is represented in the vertical section 

 of the ^%% at this stage (PI. II, Fig. 27). It will here be 

 seen that the cell-cap consists of a loosely piled cell mass, 

 of two (or three) tiers in depth, inclosing an irregular segmen- 

 tation cavity. In the floor of this blastoderm cells are dis- 

 tinguished which below open into the yolk mass, and which, 

 most important, may be seen splitting off nuclei into the 

 yolk below. This process in the formation of merocytes the 

 present writer regards as essentially the same as that occur- 

 ring in Elasmobranchs, and believes that in it the ancestral 



1 The writer has found no segmentation stages in which the furrows extend 

 much lower than the equatorial region of the egg. He does not, accordingly, 

 confirm the note and figure of Balfour, and is inclined to believe that the total 

 segmentation of Lepidosteus occurs only as a variation. 



