STRUCTURE AND DEVELOPMENT OF COET,0IT,ANA. 55 



of the internal structure fairly difficult, yet one can clearly witness that 

 the endoderm is still very simple and consists of four sac-like pouches 

 as it was in the previous stage. 



As the development of the larva still advances (figs. 11, 12), the 

 pigment spots increase very much in all the superficial parts of the body. 

 Besides yellowish ones, there appear brownish spots, particularly in 

 quantities in regions around the mouth, so that the egg now becomes 

 visible as a yellowish-brown spot even with a naked eye. Meanwhile, 

 the parts neighbouring the tentacular apparatus have grown very pro- 

 minent and marked off fairly sharply from other parts. The tentacles 

 have been elongated considerably, so as to be somewhat longer than the 

 height of the body. They are now provided with four or five branches. 

 The tentacle-sheath has grown somewhat deeper, and the gape of the 

 mouth fairly wider than before. The upper and lower halves of the 

 pharynx are demarcated sharply off from each other by the appearance 

 of a constriction between them. 



The larva of such a stage has already undergone its full deve- 

 lopment in the egg and is ready to hatch out. Such a larva is frequently 

 observed in stretching movements in the egg-membrane and in pressing 

 the mouth region against the membrane as if it attempts to free itself. 

 Eventually, the membrane ruptures at a place, whence escapes the larva, 

 directing the mouth fore wards, as does the larva of ordinary ctenophores 

 also (PI. 7, fig. 21). 



A number of larvae of this stage were fixed with Flemming's 

 strong solution, imbedded in paraffin and sectioned. Several series of 

 tolerably good sections were thus obtained, of which some are represented 

 in PI. 7, figs. 13-19, PI. 3, figs. 9-1 1. 



The epidermis shows already the definitive feature, consisting, as it 

 does, mainly of the two kinds of the gland cells described already. It 

 exhibits some topographical differences in thickness; as a general rule, it 

 is thicker in the oral, than in the aboral parts of the body. The thinnest 

 part of the epidermis exists outside of the tentacle-sheath, where it is only 

 4 fi and about ^ as thick as the thickest part near the mouth aperture. 

 The aboral sense-organ (PI. 3, fig. 9; PI. 7, figs. 13, 15, s) shows a 

 structure similar to that of the adult animal. It is a subspherical capsu- 

 lar organ made up of tall columnar ciliated cells; the nuclei are arranged 

 in the wall in three to five layers, except in parts at the bottom, where 

 they form but one or two layers in the lower half of the thickness of 

 the wall, leaving the upper half quite free of them. A few otolithic 



