144 EMBRYOLOGY 



development this cavity enlarges (Fig. 67 C, d), while the 

 mesodermal elements move upward more and more, and 

 finally spread out in the form of a plate on the inner surface 

 of the ectoderm (Fig. 67 G, me). Meanwhile the circum- 

 crescence on the part of the ectoderm has made further pro- 

 gress. Even now it not only covers the under-side of the 

 embryo, bat also grows up into the interior of the gastral 

 cavity, so that an invagination of the ectoderm (Fig. 67 C, 

 st) arises, which is comparable with the oesophagus of the 

 Anthozoa, and from which the so-called stomach of the 

 Ctenophora is subsequently developed. 



The statements of the different authors regarding the first stages of 

 cleavage are essentially in agreement with one another, but in regard to 

 the later stages, and more especially concerning the orientation of the 

 embryo, they differ somewhat, the point in question being the determina- 

 tion of the poles of the chief axis. If in the earlier stages we call that 

 pole which in our figures is the upper one the micromere pole, and the 

 oj^posite one the macromere pole, the disputed point is whether or not 

 subsequently the micromere pole becomes the sensory pole, and the macro- 

 mere pole the oral pole. We have adhered to Metschnikoff's descrip- 

 tion (No. 16), which agrees with Kowalevsky's later memoir {Literature on 

 Cnidaria in General, No. 10), because such an orientation seems probable 

 through comparison with the eggs of moUusca and worms having 

 unequal cleavage and subsequent epibolic development, and because a 

 homology of the sensory body of the Ctenophora with the apical plate 

 of these forms appears thus to be provided for. 



The development of all Ctenophora in the stages thus far described 

 appears to take place in very much the same way. Lampetia pancerina 

 alone appears, according to Chun (No. 3), to possess peculiarities, 

 especially, among others, the existence of a sixteen-cell stage, formed 

 on a strictly quadriradiate plan, etc. 



The embryo has now assumed a nearly spherical form 

 (Fig. 67 C). But the two ends of the chief axis are dis- 

 tinguished by shallow depressions. In viewing the embryo 

 from above, one recognizes that the transvei-se axis still 

 exceeds the sagittal in length. From now on a growth in 

 the direction of the chief axis is especially noticeable 

 (Fig. 68). The embryo thereby becomes more elongated. 

 Since at the same time the upper end of the body increases 

 in size, principally by the development of the tentacular 



