254 CEPHALOPODA. 



Other much larger paired prominences are found lower down at the 

 sides of the body in Loligo Pealii as the first indications of the arms 

 (Fig. 115 A, ar, Brooks). It seems to be specially characteristic of 

 this form that the rudiment of the embryo spreads over a very con- 

 siderable portion of the egg ; in other forms this is not the case, as 

 may be seen by comparing Figs. 115 A and 116 B. 



Orientation of the Cephalopod body. A few words must be said as to the 

 position in which we have represented the embryo, since it is not that formerly 

 ascribed to Cephalopodan embryos, which are generally placed with the head 

 and arms pointing upward. Our orientation of the embryo is in conformity 

 with the now universally accepted view as to morphology of the Cephalopodan 

 body, the ventral side being directed downward as is usual in other animal 

 forms. If we regard the part of the body which lies between the mouth and 

 the anus as the ventral surface, we have to consider by far the greater part of 

 the body as the dorsal surface. That end of the body which, as opposed to 

 the head appears as the posterior end must, according to this view, which was 

 first adopted by Leuckhart (No. 31), be regarded as the apex of the dorsal 

 surface. In such an assumption, according to which the ventral surface lies 

 in the horizontal line, the embryo ought really to be placed obliquely, but 

 this position was departed from for practical reasons and the head with the 

 arms was simply directed downward. The (ascending) part of the dorsal 

 surface which is directed forward will be called the antero-dorsal, and the 

 (descending) part which is directed backward the postero-dorsal surface. 

 These points are best illustrated by the median sagittal section of an older 

 embryo (Fig. 133, p. 283) which cuts through the mouth and the anus. 



In the two ontogenetic stages of Loligo (Fig. 115 A and B) as yet 

 considered, the circumcrescence of the yolk by the blastoderm is not 

 yet completed, although the rudiments of the mantle (»?), the eyes 

 (au) and indications of the arms (ar) are pi'esent. At a somewhat 

 later stage, the yolk appears completely enclosed hy the blastoderm 

 and a number of new structures appear, especially on the ventral 

 surface. Among these, the oral aperture deserves special mention ; 

 this appears as a transverse oval pit somewhat in front of or between 

 the optic rudiments, this pit arising like those rudiments very early. 

 In Fig. 116 B the mouth is seen at a somewhat later stage. 



In front of the oral aperture a swelling appears (Fig. 116 5, ar), 

 which runs right round the embryo, being divided into separate pro- 

 minences. These latter are the rudiments of the arms which thus 

 appear in Loligo vulgaris in a manner somewhat different from that 

 described by Brooks in the case of Loligo Pealii. In the latter, the 

 arms first appear in a very early stage as a pair of prominences, one 

 on each side of the body (Fig. 165 A, ar),. the individual arms 

 appearing later by the breaking up of these. There is no mention of 



