202 CEPHALOPODA. 



ring-like series in advance of the margin of the blastoderm, but 

 subsequently appearing indiscriminately over all parts of the egg. 

 They take no share in forming the epiblast, but would seem, accord- 

 ing to Lankester, to assist in giving rise to the lower layer cells, and 

 also to a layer of flattened cells which eventually completely encloses 

 the yolk, and may be called the yolk membrane. The cells of the 



yolk membrane first of 

 all appear at the thick- 

 ms ened edge of the blas- 



toderm. From this point 

 they spread inwards un- 

 der the centre of the 

 blastoderm (fig. 115 m), 

 FIG. 110. SECTION THKOUGH THE BLASTODERM and together with the 



OF A LOLIGO OVUM AT THE BEGINNING OF THE o Klie+ nolle rmtwnrrla 

 / A j?j_ T~> i j. i \ tr IJJ. Ulctio u C/t?J.lo, OLlLVVclpI Llo 



FOURTH DAY. (Alter Bobretsky.) . 



ms. mesoblast; d. cell at the edge of the blasto- over the y olk generally ; 

 derm ; c. one of the segmentation cells. SO that before long (on 



the tenth day in Loligo) 

 the yolk becomes completely invested by a membrane of cells. 



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

 layers, (1) 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 blastoderm 

 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 Grenacher (No. 280). 



In a young blastoderm of Sepia viewed from the dorsal 

 surface, a series of structures appear which are represented in 

 fig. Ill A. In the middle is a somewhat rhomboid prominence 

 which forms the rudiment of the mantle ()nt). In its centre is a pit 



