QUARTERLY CHRONICLE. 43 



left something to be clone. The ova of Sepiola are oblong in 

 shape, and contained, to the number of fifteen or sixteen, in 

 a thick mucilage. The ovum has but a single envelope, 

 Avhich is not the vitelline membrane, since it is furnished 

 with a micropyle, and must hence be regarded as a true 

 chorion. The ova are quite transparent^ and their develop- 

 ment lasts from thirty-four to thirty-five days. Three periods 

 are distinguished by the author — to the completion of the 

 blastoderm, ten days; formation of organs, five days; de- 

 velopment and completion of organs, twenty days. The two 

 lamellfe of the blastoderm form on the third day, and by the 

 eighth day its growth envelops the whole ovum. The single 

 layer of cells in each lamella execute very marked amoeboid 

 movements. At the commencement of the second period the 

 cells of the outer lamella of the superior part of the blasto- 

 derm become covered with vibratile cilia, the movements of 

 which cause a rotation of the embryo. The demarcation of 

 the foetus from the vitelline vesicle placed above it gradually 

 proceeds, and the rudiments of eyes, mantle, arms. Sec, ap- 

 pear. These organs are formed chiefly at the expense of the 

 inner lamella. The nutritive-vitellus at the end of the 

 second period j^resents a projection corresponding to the 

 mantle ; it also gives off two prolongations into the cephalic 

 sinuses, beneath the optic ganglia. The author denies that 

 this vitellus is surrounded by the jiroper membrane described 

 by Kolliker. In the third period the groAvth of the organs is 

 the chief feature. The nutritive-vitellus is absorbed little 

 by little into the body of the foetus, and finally only re- 

 presents a sort of wart upon the head between the bases of 

 the arms. The cartilaginous skeleton of the head is now 

 developed, whilst about the same time the chromatophores 

 develop in the skin, and the rudiments of the cuttle bone 

 make their appearance. The two lamellae which play so im- 

 portant a part are called by M. Mecznikow epithelial (ex- 

 terior) and parenchymatous (interior) lamella. The first 

 gives rise to the general envelope of the body, the cartilages, 

 the organs of sense and digestion, and the inkbag. The 

 inner layer gives origin to the muscles, the nervous system, 

 the mass of the pharynx, and the vascular system. These 

 lamellae correspond exactly to what M. Mecznikow has de- 

 scribed in the embryo of the scorpion. 



It appears from this that the formation of the nervous 

 system of the Sepiola cannot be paralleled with that of the 

 same system in the Vertebrata. On the other hand, the 

 formation of the skin and the organs of sense is effected, as 

 in Vertebrata, at the expense of the internal lamella. 



