No. 3.] AMPHIOXUS AND THE MOSAIC THEORY. 595 



sents a yi gastrula attached firmly by one side to an unseg- 

 mented Y2, blastomere (both within one membrane). The 

 gastrula is somewhat distorted, but is clearly of the normal 

 type. Fig. 102 shows a unilateral yi blastula, the lower pole 

 of which is turned towards the unsegmented blastomere. Fig. 

 1 01 shows a unilateral 4-celled stage, the left ^ blastomere 

 being unsegmented. The two halves are firmly united and en- 

 closed within a single membrane. The segmented half shows 

 the normal form of entire cleavage, being divided into four 

 equal cells. 



These facts, as well as those afforded by the double embryos, 

 show that tJic unity of the normal embryo is not caused by a 

 mere juxtaposition of the cells. They indicate that tJiis unity 

 is not mechanical but physiological, and point toward the con- 

 clusion that there must be a struct Jiral continuity from cell to 

 cell that is the medium of coordination, and that is broken by 

 mechanical displacements of the blastomeres. 



F. The Later Stages. 



The modified forms of development, whether isolated dwarfs 

 or multiple monsters, steadily diminish in number as the devel- 

 opment advances. Half-sized, quarter-sized, and double gas- 

 trulas are very common (6-8 hours.) Among the oval larvae 

 with notochord and 8-9 somites (Hatschek's, Figs. 49-52) half- 

 sized dwarfs are not uncommon (Fig. 142), but I have seen 

 only two or three ^ larvae, and none of these were normally 

 developed (Fig. 143).^ 



Double embryos at this stage are rare. In the individual 

 represented in Fig. 144 the two bodies are united along the 

 left side, the anterior part of each being free {cf. the corre- 

 sponding double gastrula, Fig. 68). The two notochords are 

 separate, but the archenteric cavities appear to communicate 

 posteriorly. 



1 I may call attention to the fact that Hatschek's Fig. 49 (" Kleines Indi- 

 viduum ") probably represents a Yz larva accidentally produced by manipulations 

 of the early stages. Hatschek specially calls attention to the small size of this 

 and a few other of his figures, which he says is due to individual variation. I 

 found also a considerable variation in the size of the ova, but never sufficient to 

 account for so great a difference in the larvae. 



