202 University of California Publications in Zoology. [VOL. 4 



be the case it was more likely normal, especially in view of the 

 position of the blastomeres of some eggs of the third cleavage. 

 Wilson '94 has called attention to the same thing in Amphioxus : 

 "Slight as they are," says this author, "they deserve attentive 

 consideration, for they give, I believe, a key to the more consid- 

 erable deviations of later stages. ' ' 



Third cleavage. (Fig. 5.) The third cleavage is unequal and 

 is of especial interest because of its similarity to that of Am- 

 phioxus. The blastomeres of the upper pole are smaller than 

 those of the lower. The difference in size between those of the 

 two poles is somewhat less than that shown by Wilson for Am- 

 phioxus ; otherwise the two eggs are much alike even to the 

 exception to the usual bi-lateral form of segmentation, for as in 

 Amphioxus there occurs a deviation from the typical arrangement 

 of blastomeres into the spiral form described by Wilson. 



The slight irregularity noted in the second cleavage and the 

 spiral form in the third cleavage were observed on preserved 

 material. I am therefore unable to say whether or not the irreg- 

 ular second cleavage of D. pusillus gives rise, as Wilson found for 

 Amphioxus, to the spiral third cleavage. The early cleavage 

 stages of the two animals being thus far so much alike, and also 

 in other respects to be described, suggest that they may be found 

 to be alike in this particular also. 



Fourth cleavage. (Fig. 6.) The typical fourth cleavage is 

 shown in fig. 6. Here it will be seen that the cleavage is bi-lateral. 

 All eggs observed in this stage, except one, were of this form. 

 This one exception corresponds to Wilson's mixed form of 

 cleavage. 



Fifth cleavage. (Fig. 7.) In the fifth cleavage another point 

 of similarity between D. pusillus and Amphioxus is to be ob- 

 served: the presence of a cleavage pore. This and the bi-lateral 

 form of cleavage is shown in fig. 7. A large per cent, of eggs of 

 this stage and stages immediately following possessed cleavage 

 pores. 



Sixth and subsequent cleavages. The sixth and subsequent 

 cleavages do not show clearly a bi-lateral symmetry. Two of 

 these stages are shown in figs. 8, 9, pi. 5. 



Comparison of D. pusillus with Amphioxus. Although my 



