No. 3-] AMPHIOXUS AND THE MOSAIC THEORY. 591 



Stage (Fig. 92) is more frequent than in the Yz embryo, and the 

 blastulas show correspondingly more frequent variations from 

 the typical forms. 



c. The y% blastomeres, whether large or small, divided 

 unequally at the second cleavage, in every case observed (Figs. 

 121, 126). Four micromeres are formed as usual by an 

 equatorial cleavage, but these may be equal (Fig. 127) or 

 unequal (Fig. 122). At the ensuing division (Figs. 123, 128) 

 only the two larger macromeres divide unequally, all the other 

 cells dividing equally ; and at the same time a large cleavage- 

 pore appears at the lower pole, where only two macromeres can 

 be distinguished (Fig. 124). These continue to divide 

 unequally for some time (Figs. 128-130) while the others 

 divide equally. Meanwhile the cleavage-pore undergoes a 

 varied fate. In a very large proportion of cases it enlarges at 

 each division until the embryo consists of a plate of cells (Figs. 

 129, 130) sometimes perfectly flat, more usually curved. In 

 cases of only moderate enlargement of the pore, half-closed 

 blastulas are produced (Fig. 131). In a few cases, however, the 

 pore closes completely, and then perfect closed yi blastulas 

 arise (Figs. 125, 135). 



All of these forms may live 18 hours or more, acquire cilia 

 and swim actively about. None, however, invaginate to form a 

 gastrula. (I have isolated about a score in the hanging drop, 

 and have examined many hundreds more, both living and 

 in preparations.) It is, therefore, highly probable that the 

 ^ blastomeres are unable to advance beyond the blastula 

 stage — a result in agreement with those of Roux and Driesch. 

 The yi blastulas (like the exceptional }{ blastulas observed by 

 Driesch) swim with great activity for several days. The wall 

 consists of uniform columnar ciliated cells (Fig. 135) enclosing 

 a large cavity, in which, after 24-26 hours, often appear 

 numerous clear rounded bodies that resemble the mesenchyme 

 cells of echinoderm embryos. 



C. Cleavage of Double Embryos. 



I have examined the early stages of double embryos with 

 especial interest to determine first the relation between the 



