No. 3.] SERPULORBIS SQUAMIGERUS CARPENTER. i I 7 



appear at the opposite angle. The same migration is repeated 

 in an opposite direction in preparation for the next ensuing 

 division. The appearance of the second quartette is soon fol- 

 lowed by a laeotropic division of the cells of the first. A 

 dexiotropic cleavage of the second quartette next appears, and 

 at about the same time the macromeres bud off the third quar- 

 tette in a right-handed spiral, completing the separation of the 

 ectoderm from the entoderm. The twenty micromeres com- 

 posing the ectoderm are all transparent and devoid of yolk. 

 While they form about one-half the surface of the egg, they 



D 



FIG. 2. 



FIG. 3. 



FIG. 2. Sixteen to twenty-four cell stage from the animal pole, showing the origin of the third 

 quartette and the dexiotropic cleavage of the second. The first quartette has divided, pro- 

 ducing the four " trochoblasts." 



FIG. 3. Lateral view of the twenty-four cell stage. A cleavage is taking place in the apical cells. 



form much less than half its bulk, as their thickness is not 

 nearly so great as that of the large yolk-laden entomeres. The 

 conclusion drawn by Salensky, that in Vermetus there are four 

 quartettes of micromeres produced, is doubtless erroneous. 

 The cleavage of the first quartette of ectomeres was probably 

 overlooked, and the outer products of this division regarded as 

 having had a separate origin from the macromeres. A com- 

 parison of Salensky's figures with the cleavage of Serpulorbis 

 renders this interpretation probable. Besides, there are strong 

 reasons for doubting that four generations of ectomeres are 

 ever produced among the gasteropods, as I have attempted to 

 show elsewhere. 



