58o 



TAKU KOMAI : 



the micromeres are multiplying rapidly and gradually spreading over the 

 macromeres (epiboly). Fig. 2 represents a stage in the above process, 

 seen from the micromere pole of egg, where a moderately large elliptical 

 gap exists between micromeres showing macromeres within. On the 

 macromere pole too, there is in this stage another but somewhat smaller 

 opening, which, however, is soon closed by the multiplying micromeres. 

 The gap on the micromere pole persists for some time after the closure 

 of the opening at the macromere pole, but sooner or later it too comes 

 to be closed. Directly after the closure of the gastrula at both its poles, 

 there appear first traces of the aboral sense-organ, tentacles and ribs, as 

 well as of the stomodaeal invagination, all nearly at the same time. The 

 aboral sense-organ develops on the micromere pole and the stomodaeal 

 invagination on the macromere pole, while the tentacles and ribs do so on 

 the lateral region of the gastrula nearer the micromere than the macromere 

 pole. Thus, the gastrula develops into a typical cydippid embryo. 



Cydippid Larva. — Fig. 3 shows an unhatched cydippid larva of a 



very early stage. The subspherical 

 body exhibits at its one pole the 

 mouth-opening (0) and at the opposite 

 pole the sense-organ (s), besides a pair 

 of tentacular rudiments (/) ard the 

 eight comb-plate rows (c). The sense- 

 organ, when viewed from above, is of 

 a rhomboid outline with the longer 

 diamètre in the sagittal plane of the 

 larva; the otoliths forma small aggre- 

 gation at both ends of the shorter 

 diamètre. The polar plates can not 

 yet be observed. The comb-plate 

 rows as well as the tentacle -rudiments 

 are located in the aboral half of the 

 body. The former are arranged in four close pairs, each row containing 



Fig. 3 



,8. 



•» C. 



o. 



Fig. 3. A young cydippid larva, seen 

 on the tentacular plane, x 170. 

 c. Comb-] late rows. 

 o. Mouth aperture. 

 oe. Oesophagus. 

 ph. Pharynx. 

 s. Ahoral sense-organ. 

 t. Tentacular rudiments. 



