24 



BIOLOGICAL LECTURES. 



they appear in the protoplasm of the unsegnrented egg itself 

 on each side of the animal pole. Likewise the observations of 

 Driesch and Morgan ^ on ctenophore eggs indicate what a high 

 degree of organization the unsegmented o^g^ may reach. And 

 while it is conceivable that this high degree of organization 

 of the Q.Q^^ may not lead to a highly determinate form of cleav- 

 age, yet it is to be observed that in all the cases named this 

 does happen. 



All the earlier cleavages in Crepidula are spiral, that is, radially 

 symmetrical, and this radial symmetry extends not only to the 



Fig. 3- Fig. 4. 



Fig. 3. — Crepidula, 12-cell stage ; four macromeres and eight niicromeres. 



Fig. 4. — Crepidula, twenty-five cells ; /, trochoblasts. In these and some of the following 

 figures the macromeres and first quartette are unshaded ; the second quartette is 

 stippled; the third quartette is shaded with lines ; and the fourth quartette (4d) with 

 dots and circles. The direction of the various cleavages is shown by means of arrows. 



direction and time or rate of division, but also to the size, the 

 position, and the histological character of the resulting blasto- 

 meres. The result is a number of radial structures such as the 

 four trochoblasts (Fig. 4, t), the four arms of the ectoblastic 

 cross (Fig. 5 et scq.), and the four rosette series of cells 

 (Figs. 10, 12), some of which give rise to certain radial 

 structures of the larva. Not a single bilateral cleavage ap- 

 pears up to the 44-cell stage, and radial cleavages generally 

 prevail throughout the Qgg until a much later period. In all 

 cases bilateral cleavages first appear in certain cells on the 

 posterior side of the egg and in processes which lead to the 

 elongation of the body along the posterior axis. This bilater- 



1 Driesch und Morgan, " Zur Analysis der ersten Entwicklungsstadien des 

 Ctenophoreneies," Arch.fiir Entwickhmgsinechanik, Bd. 2, 1895. 



