BY H. L. KESTEVKN. 679 



A section of tlie four eelled-einbryo is shown in Fig.^-i, anil the 

 reconstructed embryo in Fig. 11. 



Fig. 34 gives one the idea that there has been unequal segmenta- 

 tion; this, however, is due to the fact that the section is not abso- 

 lutely at right angles to the long axis. 



Cleavage next takes place twice successively at right angles to 

 the long axis, resulting in the eight- and sixteen-cell stages. 



In the sixteen-cell stage, the embryo still retains its egg-shape, 

 though it has increased slightly in size, owing to the separation 

 of the blastomeres to give rise to a blastocoel (Figs. 35 and 36), 

 Fig. 37 is a longitudinal section through a compacted embryo, in 

 which the blastocoel has not yet appeared. 



The appearance of the embryo in the sixteen-cell stage is shown 

 in Fig. 12. 



The rotational movement of the polar blastomeres, which leads 

 to the even capping of the poles, is well shown in Fig. 35. 



The fifth cleavage is again parallel to the long axis, and results 

 in the thirty-two celled embryo. The blastocoel has increased a 

 good deal in size (Fig. 38). 



There now follow two more total and equal cleavages, the first 

 in the longitudinal plane, the second in the transverse plane. These 

 cleavages result in the formation of the hollow blastula. 



There next follows a cleavage which does not affect a few cells 

 situated at the posterior pole, about eight in number. The blastula 

 is further enlarged at this division by increase of the blastocoel. 



The larger cells, at tlie posterior end, now divide actively, their 

 daughter-cells being shed into the blastocoel, where they, in turn, 

 divide, to give rise to a solid mass of small cells, more or less com- 

 pletely filling the blastocoel(Figs. 39, 40, and Fig. 14). 



Meanwhile, the parietal cells have undergone further division, 

 and the embryo has decreased in size. 



In this stage, the embryo is a solid gastrula, gastrulation having 

 been by a much modified involution. The blastopore is represented 

 by a saucer-like depression at the posterior pole ( Fig. 13 ) . 



Histogenesis and Organogeny. — In the hollow blastula, those 

 eight cells, the budding of which is to give rise to the solid core of 



