12 AET. 11. — S. HATTA. 



conical eminence decreases in height, the groove grows further 

 in curvature, for it is pushed forwards on the dorsal surface of 

 the ovum by the eminence now flattened in consequence of 

 withdrawal of the cells composing it. As the invagination goes 

 further, the elevation increases in its antero-posterior extent, 

 pushing the groove still further forwards. The eminence may 

 now be called the embryonic shield {Figs. Qa, and Q>b, em.s.). 

 By the time when the invagination has extended over the larger 

 part of the vegetative field {Fig. 7c.), the ovum is completely 

 surrounded by the groove, although still shallow in the ventral 

 part just constricted {Fig. 7a, bg.). 



The translucent part of the ovum, against which the remain- 

 ing parts are marked ofiP by the groove, is in the course of 

 development, continually diminishing in extent, and at a stage 

 like that represented in Figs. 8a and ^b, it is reduced into a 

 small swelling, while the embryonic shield has extended consider- 

 ably. This reduction of one part and the augmentation of the 

 other are, in early stages, evidently the effects of one and the 

 same process, i. e., of the invagination, by wdiich the segmenta- 

 tion cavity is partially obliterated ; but in later stages, the 

 matter is much complicated, as will be explained in the following 

 lines. 



I will here call attention to the change of the position of 

 the groove in the stages spoken of. A comparison of the 

 groove in Fig. 7a with that in Fig. 8a shows an apparent 

 forward shifting of it, i. e., an extension of the embryonic shield 

 {em.s.). If this shifting of the groove were done, as in early 

 stages, merely by the sinking down of the opaque hemisphere 

 from the surface, it ought necessarf'ily follow that the junction- 

 line at the ventral median line (indicated with l) would have much 



