10 J M. U.ssow's Zoologico-Einhrifohyical Investigations. 



commence outside the body of tlic parent. The segmentation 

 of the formative vitclhis of the Cephahipoda greatly reminds 

 us, as regards its form, of the segmentation of the eggs of 

 birds* and Cheloniaf. In all the four species of Cephalo])oda 

 investigated by me it is irre(/ular. The division of the pro- 

 toplasm of the formative vitellus commences in its thickened 

 central part, and spreads towards the attenuated peripheral part, 

 which uniformly surrounds the lohole surface of the nutritive 

 vitellus. The latter takes no part in the segmentation pro- 

 cess ("partial segmentation"). One of the chief causes of 

 the segmentation of the formative vitellus seems to be the 

 great mobility of its protoplasm, and the changes of position 

 of its heaviest parts, the darkest-coloured granules. The 

 segmentation always begins in the vicinity of the nuclei of 

 the segmentation-cells (spheres of segmentation) or segments ; 

 and the close of the complete cleavage (by longitudinal or 

 afterwards transverse division) coincides with the complete 

 separation of the nuclei. At first all the cleavages appear only 

 at the surface of the formative vitellus, but then gradually 

 penetrate by deepening to the lowest layers of the proto- 

 plasm. 



The original or first furrow J, which divides the whole 

 formative vitellus into two equal segments lying side by side, 

 is soon (in about two hours) intersected at right angles by a 

 second furrow. As the result of this division four equal seg- 

 ments, enclosing four clear nuclei, are produced (the nucleoli 

 are entirely deficient). In the central point there is produced 

 a very inconsiderable clear interspace, which in the sequel 

 soon disappears. The subsequent cleavages of the formative 

 vitellus are in-egular ; from four segments there are formed 

 (in four hours), first six^ and then eight equal segments. In 

 the period between the formation of the six and of the eight 

 segments, there are produced at the centre of union of the 

 furrows, in the earliest moments of the appearance of the two 

 narrowest segments, by constriction of the apices of these, two 

 primitive cells or spheres of segmentation (approximately be- 

 tween the third and fourth hour of the process of segmenta- 

 tion). From the two of the eight segments which are situated 



* Coste, Hist. part, et gen. des corps orfranis^a, p. 287, pi. ii. 



t Agassiz, Contrib. to the Nat. Hist, of the United fStates, ii. 



X In Loligo,Sepiola, a,nd Ar(/onauta this fun-ow appears directly beneath 

 the micropyle, in the centre of the formative vitellus ; in Si'pia sometimes 

 a little to one side, which I repaid as an abnormal phenomenon, as also 

 that I once in Sepio/a found the sopmontation on tlie lower obtuse pole of 

 the ovum. The horns mentioned in the following description of the pro- 

 cess of segmentation relate to Sepiola and LoUgo. 



