2 24 Edmund B. Wilson. 



The constant result of this experiment was, that while both quar- 

 tets produce closed hlastulas, only the lower one gastrulates, while 

 only the upper one produces an apical organ. 



6. Summary on the first quartet. 



The foregoing observations are sufficient, I believe, to establish 

 definitely the mosaic character of cleavage and differentiation in 

 the first quartet. This is strictly proved for the primary trocbo- 

 blasts (i^), for their first (i"-^ and i'-) and second (I'-^'S I^•^■^ 

 etc.) products, and for the i^ cells; it is less strictly but hardly less 

 convincingly established for the apical cells and the secondary 

 trochoblasts. Excepting the secondary trochoblasts, the other 

 products of i^ do not show sufficiently definite characters to allow 

 of a similar definite proof, but it can hardly be doubted that the 

 same conclusion applies to them, and also to the first quartet taken 

 as a whole. 



B. EXPERIMENTS ON CELLS OF THE LOWER HEMISPHERE. 



Isolated blastomeres of the lower hemisphere are in general 

 much less tenacious of life than those of the upper quartet (a fact 

 parallel to that observed by Driesch in sea-urchins), and my ob- 

 servations are here much less detailed. In a more general way, 

 however, the results are entirely in agreement with the conclu- 

 sions reached in case of the upper quartet. 



I. Development of the isolated Ys-macromere. 



These cells divide, at least as far as the 64/8-cell stage, as if 

 forming part of a complete embryo. At the first division a cell 

 of the second quartet is formed (Fig. 69-70), which at the en- 

 suing cleavage divides nearly equally (into 2^ and 2-), while a 

 cell of the third quartet is produced from the basal in the proper 

 position (Fig. 71). At the next cleavage 2^ divides nearly 

 equally, 2" very unequally, forming below the characteristic small 

 cell 2--, that lies against 3. The latter cell then divides equally 

 or unequally (the quadrant probably being in the former case one 



