252 CEPHALOPODA. 



whole of the blastoderm or may at first be found only at definite 

 parts, appearing, for instance, especially at the growing edge of 

 the blastoderm (Figs. 115 and 116, p. 255). If this ciliation develops 

 strongly and spreads over the whole of the blastoderm which covers 

 the greater part of the yolk, as is the case in Loligo, the embryo 

 rotates within the egg-integument, a phenomenon which recalls the 

 free-swimming larvae in other divisions of the Mollusca. In Argo- 

 nauta and Octopus, the only movement that takes place in the embryo 

 is the shifting of the pole at which cleavage began away from the 

 micropyle-region to that of the egg-stalk (KAY LANKESTER). Some 

 of the embryos of Octopus are, in fact, found lying at the micropyle 

 end of the egg and others at the opposite pole ; in the older embryos 

 examined by us the last position was the more frequent. 



The ciliation of the embryo either soon disappears, or else is 

 retained for a long time, as, for instance, in Sejna, where it is found 

 both in the embryonic region, which is already far developed, and on 

 the yolk-sac. The embryos of Sepia, however, notwithstanding this 

 ciliation, do not rotate like those of Loligo, a fact evidently due to 

 the large amount of the yolk (KOLLIKER). 



Even before the circumcrescence of the yolk by the blastoderm is 

 completed, indications of the future shape of the Squid appear in the 

 blastoderm or germ-disc (Figs. 115 and 116 A). We shall describe 

 the rudiment and further development of the body-form at first in 

 one of those species in which the blastoderm grows round the yolk 

 very early and the embryonic rudiment at first encloses the greater 

 part of the yolk. The ontogeny of such a form, Loligo Pealii, was 

 studied in a very thorough manner by BROOKS (No. 7). RAY LAN- 

 KESTER has also published investigations as to the ontogeny of 

 Loligo (No. 30) and representatives of this genus, as has already been 

 mentioned, were studied by earlier zoologists (P. van BENEDEN, No. 

 3 ; METSCHNIKOFF, No. 32). Our own investigations, made with 

 very rich material of Loligo vulgaris, as well as Octopus milgaris and 

 Argonauta have enabled us to supplement the discoveries of earlier 

 observers. 



A. Development of the Embryonic Rudiment through its 



extension over the greater part of the egg with 



subsequent development cf a Yolk-sac. 



(a) Loligo. 



In the region in which cleavage began, i.e., at the animal pole of 

 the egg, a swelling forms in consequence of the thickening of the 



