400 EDWIN G. CONKLIN 



I have devoted especial attention to the character of the larvae 

 derived from single blastomeres. 



There are many variations of structure in such larvae, some 

 being more defective than others. In cases where the gastrula 

 remains uncovered by ectoderm on the injured side, the edges 

 may fold in toward one another in later stages, until they come 

 into contact, thus closing the open side and bending the body of 

 the embryo, as shown in fig. 4. Here the larva is clearly a f 

 larva, although by this bending some cells at the edge of the open 

 side are displaced more or less to the injured side. In many 

 cases however ^ larvae are completely surrounded by ectoderm, 

 and they are generally more solid than typical larvae, i.e., they 

 contain few if any cavities (figs. 5, 8). The cells of the neural 

 plate may form a thickened mass but they never surround a 

 neural canal. On the other hand a gastric cavity may be present, 

 but it is usually very small (figs. 5, 8). The notochord presents 

 a nearly typical appearance, being composed of disk-shaped 

 cells which are disposed in a single series (fig. 4). On the other 

 hand paired structures, such as muscle cells and mesenchyme 

 are found only on one side, where they are frequently typical in 

 appearance (figs. 4-6). 



But while many larvae are thus plainly defective there can be 

 no doubt that some of the | larvae of Phallusia are frequently 

 strikingly like normal ones. They may be rounded and entirely 

 covered by ectoderm, there being no opening on the side next 

 the injured half; they may have a well-marked body and tail; 

 they may possess a typical notochord, an eye spot, and may 

 have a gastric cavity surrounded on all sides by endoderm cells 

 (figs. 9, 10). Such I larvae of Phallusia are much more typical 

 in appearance than are those of Cynthia, and it is not surpris- 

 ing that Driesch, who studied them only in the living condition, 

 regarded them as typical and entire, barring a few minor defects. 

 Among such evident defects may be mentioned the fact that the 

 neural plate never infolds in these | larvae, and the eye pigment 

 is more or less scattered in surface cells (figs. 5 and 11), con- 

 ditions which sometimes occur among entire and otherwise 

 normal larvae (p. 397). Even in the study of serial sections it is 



