VII MOLLUSCA— ONTOGENY 265 



cerebral, pedal, and visceral ganglia, and the auditory vesicle appear during the 

 pai'asitio stage, in the same way as in other Lamellibranchs. 



During the last week of parasitic life the capsule formed by a growth of the 

 tissue of the host which surrounds the embryo becomes thinner ; the parasite breaks 

 through it, and falls to the bottom of the water as a young Mussel. The only 

 organs still wanting are the genital organs, the outer leaves of the gills, and the oral 

 lobes. 



£. Cephalopoda. 



Tetrabranchia. — Nothing is known of the development of Nautilus. 



Dibranchia. — The egg is usually very large, and contains, like that of the sharks, 

 reptiles, and birds, a great quantity of nutritive yolk. It belongs to the telolecithal 

 meroblastie type, and is enclosed in a capsule. A number of such capsules may 

 become cemented together to form strings. The partial segmentation takes place 

 at the animal pole of the egg, and leads to the formation at that point of a germinal 

 disc (blastoderm). 



Ontogeny of Sepia. — The blastoderm grows so very slowly round the yolk, that 

 long after all the outer organs of the embryo are quite recognisable in the region 

 of the original germinal disc, the opposite pole is still occupied by the yolk. The 

 germ lies in such a way that the centre of the germinal disc or animal pole is placed 

 dorsally, and corresponds with the uppermost point of the visceral dome of the adult 

 animal, while the mass of nutritive yolk lies ventrally. 



1st Stage (Fig. 221 A). — In the centre of the germinal disc there appears an 

 oval-rhombic bulging ; this is the rudiment of the visceral dome and the mantle. 

 On each side of this there arises a bean-shaped prominence, the rudiment of the eye. 

 Behind the eye, on each side, a long narrow ridge rnns backward in a curve ; about 

 half way down this ridge a small prominence, the rudiment of the funnel cartilage, 

 forms close to its outer side. The part of the ridge lying in front of this prominence 

 becomes the muscle which runs from the funnel to the nuchal cartilage ; the 

 posterior part (which lies behind the rudiment of the visceral dome and mantle) 

 forms the paired rudiment of the funnel itself. Between the two rudiments of 

 the funnel two other prominences rise symmetrically — the rudiments of the gills. 

 A pit in the centre of the rudiment of the visceral dome has been indicated as the 

 rudiment of a shell gland (?). 



2nd Stage (Figs. 221 B and 222 A). — The rudiments just described stand out 

 more prominently. On the outer and posterior sides of the rudiments of the funnel 

 the rudiments of the two posterior pairs of arms first appear as prominences, then 

 those of the third and fourth pairs. The first indications of the head are seen in 

 the form of a large double swelling on each side, the outer and anterior part of 

 which carries on each side the rudiment of the eye. The embryo becomes covered 

 with cilia. At the extreme anterior end the mouth appears in the middle line, 

 forming the opening of the oesophagus, which begins to sink inwards. 



3rd Stage (Fig. 221 C). — The whole embryo has become more arched dorsally, 

 and more marked off' from the yolk. On the latter, the blastoderm, which consists 

 of two layers, an external ectoderm and an internal yolk membrane, has spread out 

 further towards the ventral (vegetative) pole of the egg. At the posterior edge of 

 the rudiment of the visceral dome, the mantle fold has grown out forward in such a 

 way as to form a small mantle cavity, which already partly covers the rudiments of 

 the gills. In the space between the rudiments of the funnel and the gills the 

 proctodseum has formed by invagination, and its aperture, the anus, can be seen. 

 The rudiment of the fifth pair of arms appears. 



4th Stage (Figs. 221 D and 222 F, G). — The visceral dome projects further. 



