30 ORGANIZATION AND CELL-LINEAGE OF ASGTDIAN EGG. 



maturation are uniformly reported to take place at the animal, i.e., at the more 

 richly protoplasmic pole, whereas in Ciona they take place at the vegetative pole. 

 . . . The statement made in the preceding paragraph presents a condition of affairs 

 so directly contrary to that found in other groups of animals, as well as to what has 

 been assumed by all previous writers to he the case in ascidians, that it requires the 

 presentation of unmistakable evidence in its support. Such evidence I have to offer, 

 both from the study of the living egg and from that of preparations." 



What is this evidence? So far as it relates to the origin of the polar bodies at 

 the vegetal pole it is twofold; (a) the polar bodies are formed at the yolk-rich 

 pole, (b) this pole becomes the endodermal pole of the gastrula. As to the first of 

 these propositions I have already shown that the germinal vesicle fades and the first 

 maturation spindle appeal's at the protoplasmic pole (tigs. 77, 78, 172). Only later, 

 after the entrance of the spermatozoon, does the protoplasm now away from this pole. 

 leaving the maturation spindle closely surrounded by yolk; still later, during the 

 first cleavage, the protoplasm Hows back again to near the center of the egg and at 

 the (dose of this cleavage it moves still nearer to the pole at which the polar bodies 

 lie (figs. 100, 102. 10(1. 107. 178); thereafter this pole is always the more richly 

 protoplasmic. Therefore, except for a brief period after the fertilization and before 

 the first cleavage, when the protoplasm is temporarily withdrawn from the matura- 

 tion pole through the influence of the spermatozoon, the maturation or animal pole 

 and the more richly protoplasmic pole are one and the same in ascidians as in other 

 animals. 



As to the statement that the polar bodies are formed at a point which corresponds 

 to the center of the dorsal or endodermal pole of the gastrula it is evident that 

 unless the polar bodies have been actually followed through the development to a 

 stage when the ectodermal and endodermal poles are unmistakable, this statement 

 must rest upon indirect evidence furnished by a study of the cleavage stages. As 

 a matter of fact, Castle has not figured nor described the polar bodies in any egg 

 later than the 1G to 24-cell stage, whereas there is no trace of gastrulation in Ciona 

 before the 76-cell stage (fig. 200). Undoubtedly therefore Castle's evidence that the 

 polar bodies are formed at the endodermal pole must be indirect rather than direct, 

 and must he derived from the study and orientation of the cleavage stages. We 

 may therefore turn at once to the evidences which led him to reverse Van Beneden 

 and Julin's orientation of these stages. So far as 1 am able to discover there are, in 

 addition to several minor considerations which could at best be considered only as 

 confirmatory, two and only two general lines of evidence which he brings forward 

 in favor of his contention. They are the following: 



(1) The hemisphere in which division is earliest as the egg passes from the L6- 

 cell stage to the 32-cell stage, and from the latter to the 46-cell stage becomes later 

 the ventral or ectodermal hemisphere of the embryo ( 1S94, p. 206 ; 1896, pp. 229 and 

 235). The second paper refers to this proposition as having been demonstrated in 

 the first. What is this demonstration'.' So far as 1 can ascertain it consists merely 

 in the assumption that the cells, which in the lG-cell and 32-cell stages divide earlier 



