64 C. 0. Whitman 



represented in fig. TS and fig. 91 , has fully satisfied me that the 

 broad end of the pyriform embryo (upper end in the figures) corre- 

 sponds to the cephalic pole of the adult; in other words, that the main 

 axis of the Gastrula coineides , with that of the complete animai . If 

 Julin's conclusion be correct, then we have to admit a remarkable dif- 

 ference in two cycles of developmeut otherwise closel}' similar. 



The Or ig in of the Germ-cells. — The first gerni-cell 

 appears after the clase of the Gastrula. about the time the pyriform stage 

 is reached. The nucleus , of the central cell undergoes a karyolytic 

 division (figs. 8 and 85), the plane of division being at right angles to 

 the axis of the embryo. Two nuclei are thus formed, the larger of 

 which takes nearly a central position in the mother cell, the smaller 

 lies near the bind end of this cell. I can not say whether during the 

 division of the nucleus a portion of the protoplasm of the central cell is 

 actually split off, or whether the protoplasmic body of the germ-cell 

 forms eudogenously. 1 bave seen no case in which the nucleus of the 

 germ-cell was free, and it seems probable that immediately »after its 

 formation an area of protoplasm becomes deliaiited around it. The 

 body of the germ-cell differs in its appearance from the protoplasm of 

 the central cell , being less granular and somewhat darker '. The sec- 

 oud ol' the primary germ-cells makes its appearance at the fore end of 

 the central cell, or, as we may now call it, the axial cell (fig. 89). I 

 have seen but one case in which the second germ-cell lay behind the 

 central nucleus. Although 1 have not succeeded in tracing the origin 

 of this cell to the nucleus of the central cell, I think there can be little 

 doubt that it arises in precisely the same manner as the first. The cen- 

 tral nucleus takes no further share in the formation of germ-cells. From 

 these two primary germ-cells arise all the germ-cells that appear later 

 in the axial cell. 8ometimes before, but generally soon after birth, 

 multiplicative division begins, each of the primary germ-cells dividing 

 into two (fig. 95) . Frequently, perhaps always, the posterior cell divides 

 first (fig. 94). After each division the resulting cells consume some 

 time in growth before again dividing. In fig. 96 , the posterior pair of 

 small cells bave arisen by the division of a cell like the large cell imme- 

 diately in front of them. In fig. 97. two such pairs of cells are seen. 

 Thus there is a slow mnltiplication of germ-cells going on during the 

 growth of the embryo into the mature form. 



1 These appearances remind me of the formation of germ-cells ìd Triton, as 

 described by Mr. Iwakawa of Tokio (36, p. 267). 



