50 T. KOMAI: STUDIKS ON TWO ABERRANT CTP:N0PII0RES 



the macromeie pole that is closed first and develops the stomodaeal 

 invagination, just as it was given by Ciiux ('80) for the development of 

 Beroe. 



Towards the end of the gastrulation, the shape of the entire zgg, 

 which has been somewhat flattened up to this time, again becomes 

 globular (figs. 18-21). For some time after the closure of the opening 

 of the macromere pole, there remains still a large aperture at the 

 micromere pole which leads into a roomy axial space (figs. 18, 19). 

 This space diminishes in width gradually, the change beginning near 

 the macromere pole and advancing towards the micromere pole, so that 

 at a period during the change, the space may show a funnel-shape 

 (fig. 20). Meanwhile, the layer of micromeres on the surface of &g^, or 

 the ectoderm as it may be called now, has given rise to a thickening 

 at the macromere pole (fig. 21, stin), which represents the first trace of 

 the stomodaeal invagination. As development still goes on, the opening 

 at the micromere pole is also closed, and at the same time the axial 

 space is reduced into a narrow longitudinal slit throughout the length, 

 except in a small part at the middle where it persists as a moderately 

 broad cavity (figs. 22, 23). Shortly after the closure of the opening of 

 the micromere pole, there appear the first traces of the aboral sense- 

 organ, tentacles and ribs nearly simultaneously, all as ectodermal 

 thickenings. The aboral sense-organ develops from the micromere pole, 

 while the tentacles and ribs make their appearance on the lateral sides 

 nearer the micromere than the macromere pole. 



The aboral sense-organ (figs. 25, 26; s) arises on that pole as an 

 ectodermal thickening made up of columnar cells. It sinks gradually 

 below the general surface of the body and forms a globular capsule. 

 The formation of the otoliths takes place somewhat later than the 

 appearance of the comb-plates in ribs. The first traces of the ribs are 

 discernible at a fairly early period (fig. 22, r), in fact even somewhat 

 prior to the closure of the opening of the micromere pole, as four 

 streaks of a peculiar kind of ectodermal cells of a small size and granular 

 appearance radiating from that pole and traversing the surface of the 

 gastrula. The formation of the stomodaeum as a distinct invagination 

 takes place somewhat later than the ai)pearancc of the rudiments of ten- 

 tacles and ribs, although the ectodermal thickening which is destined to 

 develop into it arises directly after the closure of the opening of the 

 macromere pole. 



As development still goes on, the ectodermal thickenings of the 



