The Ascidian Half-Embryo. 55 



to a half of the normal 24-celled stage. This is so, for the reason 

 that further passive rearrangements of the cells occur, obscuring 

 the partial character of their origin, and causing the cell complex 

 in its solid, or " complete," condition to resemble a normal or 

 " complete " embryo. Nevertheless, the succession of rhythmic 

 cleavages, relation of successive cleavage-planes, etc., point to the 

 operation of factors which are counterparts of those operating in 

 a half of the normal embryo. 



Later development. The embryo is now " complete," and gives 

 rise to a complete blastula and larva. Although the process of 

 gastrulation has not been carefully observed, enough of the later 

 development has been ascertained to prove that a larva arises 

 which resembles the normal larva, except as regards its smaller 

 size and certain minor defects. My results, therefore, are en- 

 tirely confirmatory of those of Driesch upon Phallusia. 



C. — Cleavage of the £ blastomeres. — One of the isolated blasto- 

 rneres of the four-cell stage, is divided at the next cleavage by a 

 plane which is seen to be at right angles to both of the preceding 

 planes. Therefore it corresponds to the third cleavage plane of 

 the normal embryo. The §■ stage is shown in fig. 17. A subse- 

 quent cleavage cuts each of the cells equally, and a -^ stage re- 

 sults (figs. 18, 19), until this time, one is left in doubt as to the 

 true nature of the fragment, that is, whether it will segment as 

 a quarter or as an entire egg. However, from this time on, the 

 character of cleavage is exactly that of a quadrant of a normal 

 embryo. 



When division next occurs, only the two cells toward the ob- 

 server segment (fig. 20), and a stage of six cells results, which is 

 evidently comparable to a ^ embryo only, and not to any stage 

 of the normal development. After a normal period, the dorsal cells 

 (lower in the figure) pass into the sixth generation, and an -^ 

 ernbryo (fig. 21) is the result. As in the previously described 

 fragments, passive rearrangements occur when the resting condi- 

 tion is assumed, and the cells flatten down upon one another 

 (fig. 22). The cells of the ventral half segment at the next period 

 of activity (fig. 23), while the dorsal cells remain undivided. The 

 resulting if stage, although solid, is nevertheless derived from 

 the ^.blastomere through a segmentation of a partial character. 

 This partial character is expressed chiefly in the characteristic 

 rhythm of cleavage. 



