1904.] NATURAL SCIENCES OF PHILADELPHIA. 695 



In the 4-cell stage of the Dinophilus ovum but one polar furrow 

 is usually present, that at the animal pole, formed by the junction of 

 A and C. This furrow is very long and turns to the right when seen 

 in the second cleavage plane. At the vegetal pole all four cells meet 

 at a point (figs. 6 and 7, text fig. II, A and B). The condition where 

 a polar furrow exists at the animal pole is also found in the Lamelli- 

 branchs mentioned alone ; in these, however, the furrow is formed by 

 the junction of B and D. The phenomenon of a long polar furrow 

 existing at the animal pole in Unio is explained by Lillie (1895) as 

 being due to the fact that "the greater mass of the first blastomeres 

 is ectodermal." This explanation fits the case of Unio very well, since 

 the blastomere D which contains the greatest mass of ectoderm takes 

 part in the formation of the polar furrow, but does not explain the 

 situation in Dinophilus, where D is entirely excluded from the polar 

 furrow. This condition is probably due to two factors: (1) the ex- 

 treme obliquity of the second cleavage spindles; and (2) to the rela- 

 tively small size of B. The inclination of the spindles in A-B and C-D 

 cause A and C to lie above B and D and so meet in a long cross furrow. 

 Were B as large as either A or C it would meet D in a furrow at the 

 vegetal pole, but since its mass is so much less than that of the other 

 cells it just touches D at the vegetal pole. 



The somewhat complex relation of the first and second cleavage 

 planes to the embryonic axis will be discussed in a later section; for 

 the purpose of convenience in description, however, the blastomeres 

 A and B will be considered as anterior, C and D as posterior. 



(3) Segregation of EctoUast. 4-26 cells. 



Immediately after the second cleavage of the ovum the nuclei of 

 the blastomeres resume mitotic activity. In fig. 6 asters are seen in 

 all four cells, while between A and C there are still the remains of 

 spindle fibres to be seen. In D the asters have assumed nearly their 

 definitive position, indicating where the ends of the dexiotropic spindle 

 are to lie. 



At this time there comes to light a curious and striking feature of 

 the cleavage of the Dinophilus egg, mentioned by Lang (1884) as occur- 

 ring in Discocoelis, by Lillie for Unio (1895) and by Jennings for 

 Asplanchna (1896), also by Child (1900) as occurring occasionally in 

 Arenicola at the third cleavage. This peculiarity consists in the fact 

 that the macromeres never divide simultaneously, but always succes- 

 sively and in a regular and invariable order. This order is the same 

 as that of Unio and Arenicola, i.e., D, C, A, B, while that of Asplanchna 



