244 CEPHALOPODA. 



blastoderm. From this point they spread inwards under the centre 



of the blastoderm (fig. 



115 tti), and, together 



with the epiblast cells, 



outwards over theyolk 



generally ; so that 



before long (on the 



tenth day in Loligo) FIG. no. SECTION THROUGH THE BLASTODERM 



jay "5 V J op A LoLIGO OVUM AT THE BEGINNING OF THE 



the yolk becomes com- FOURTH DAY. (After Bobretzky.) 



pletely invested by a ms. mesoblast ; d. cell at the edge of the blasto- 



i r 11 derm; c. one of the segmentation cells. 



In the non-germinal region the blastoderm is formed of two 

 layers, (i) a flattened epiblast, and (2) the yolk membrane. In 

 the region of the original germinal disc the epiblast cells become 

 columnar, and below them is placed a ring of lower layer cells, 

 which gradually extends towards the centre so as finally to form 

 a complete layer. Below this again comes the yolk membrane 

 just spoken of. 



Before describing the further fate of the separate layers it is 

 necessary to say a few words as to the external features of the 

 embryo. In the adult Cephalopod it is convenient, for the sake 

 of comparison with other Mollusca, to speak of the narrow space 

 enclosed in the arms, which contains the mouth, as the ventral 

 surface ; the aboral apex as the dorsal surface ; and what is 

 usually called the upper surface as the anterior and the lower 

 one as the posterior. 



Employing this terminology the centre of the original blasto- 

 derm is the dorsal apex of the embryo. In the typical forms 

 with a large yolk-sack the whole embryo is formed out of the 

 original germinal disc ; the part of the blastoderm which is 

 continued as a thin layer over the remainder of the egg forms a 

 large ventral yolk-sack appended to the head of the embryo. 

 The following description applies especially to two types, which 

 form the extremes of the series in reference to the development 

 of the yolk-sack. The first of these with a large yolk-sack is 

 Sepia, of which Kolliker in his classical memoir (No. 281) has 

 published a series of beautiful figures. The second, with a small 

 yolk-sack, is the pelagic larva of an unknown adult described by 

 Grenadier (No. 280). 



