592 WILSON. [Vol. VIII. 



cleavage of an isolated Yt. blastomere and that of a normal 

 cleavage-half, and second the period at which the axes of the 

 twin bodies are determined. The results demonstrate (i) tJiat 

 the blastovieres of the Jialf-sized embryos cannot be identified 

 individually with those of half a normal embryo ; and (2) that 

 the axes of the twin bodies are determined with the first division 

 after the operation. 



A series of forms exists among the double cleavage-stages 

 that shows as obvious relation to a corresponding series among 

 the double gastrulas, though I have never in a single instance 

 succeeded in tracing the connection in the living state. In the 

 simplest case (Fig. 112) the i6-celled stage shows but a slight 

 lateral extension, corresponding with the laterally expanded, 

 but still undivided gastrulas (Figs. 64, 66). From this case 

 every transition exists to an obviously double 8-celled stage 

 (Fig. 114) in which the embryo clearly consists of two 8-celled 

 stages, adherent along the line of first cleavage and with 

 parallel axes. Such forms probably give rise to gastrulas like 

 that shown in Fig. 71, in which the axes are parallel and the 

 blastopores turned in the same direction. In still more highly 

 modified cases (Figs, in, 118, etc.) the axes of the two halves 

 form various angles, clearly foreshadowing such conditions in 

 the gastrula stage as are shown in Figs. 70-76. 



A study of the earlier stages shows that the axial relations 

 are already definitely established in the double 2-celled and 

 4-celled stages. This is clearly shown in Figs. 106-110. In 

 Fig. no (double 4-celled stage) the axes are vertical and 

 parallel, while in Fig. 109 they form an angle. In Fig. 107 

 (double 2-celled stage division) they are nearly at right angles ; 

 in Fig. 106 they form an acute angle. A study of these 

 various forms leaves no doubt that the direction of the axes is 

 deter'tnined with the first division of the displaced blastomeres, 

 which (as I have shown at p. 590) stands in definite relation to 

 their longest diameter, and this in turn is determined by 

 the shape of the cells before displacement. These facts seem 

 to justify the conclusion that the axial relations of the twin 

 bodies is purely fortuitous. The first cleavage of each blasto- 

 mere is in the same plane it would have followed in the normal 



