56 The Ascidian Half-Embryo. 



Concerning the later stages, the results of Driesch are again con- 

 firmed. The young larvae represented in Figs. 25,26 of this paper 

 illustrate one point further, although of minor consequence. It 

 will be seen that the long axis of the £ larva in fig. 25, and the 

 long axes of the ^ larvae derived from the same egg, in fig. 26 > 

 are approximately parallel to the principal dorso-ventral-axis of 

 the original egg. 



Summary and Conclusion. 



An isolated blastomere of the Molgula egg segments as if still 

 forming a corresponding part of an entire embryo. The cleavage 

 phenomena are strictly partial, as regards the origin of cells, 

 the inclination of cleavage-planes, and especially in respect to 

 the rhythm of segmentation. The general appeai'ance of the frag- 

 ment differs materially from that of a half of a complete embryo, for 

 the reason that rearrangements of the blastomeres occur, which 

 tend progressively to mask the partial nature of development. 

 The end result is a larva of less than normal size, and with defects 

 in certain of its systems. These defects are undoubtedly due to 

 the fact that but a portion of the normal amount of material is 

 available for the formation of the larva; that, for instance, the 

 chorda of a larva derived from a one-half blastomere, receives but 

 one-half of the normal number of cells, and consequently a chorda 

 of one row, and not two rows of cells, results. 



In conclusion, one is constrained to adopt the view of Roux- 

 namely, that in Molgula as in the well-known case of the echino, 

 derms (Driesch, Wilson, and others) the development begins as 

 a partial one, but that the missing part is gradually supplied by 

 the cells already present. Driesch is also entirely correct, as far 

 as the end result, a nearly complete larva, is concerned. 



Explanation op Plate IY. 



Magnification of figs. 1-3 about 280 diameters ; of all other figures, 250 

 diameters. The arrows show the direction of cleavage. 

 Fig. 1, 8-cell stage of Cioila from Castle (fig. 23), from the left side. 

 Fig. 2, 16-cell stage of Ciona from Castle (fig. 24), from the left side. 

 Fig. 3, 24-cell stage of Ciona from Castle (fig. 43), from the right side. 

 Figs. 4-8, cleavage of the left % blastomere of Molgula, from the side. 

 Fig. 4, | embryo. 

 Fig. 5, | embryo. 



