Miscellaneous. 451 



rudiments of the eyes. At the same period there appears on one 

 side of the ovum a small fold of the outer lamella, which speedily 

 extends all round the embryo, and constitutes the rudiment of the 

 mantle. On the second day, also, the mouth shows itself on the 

 ventral surface as a depression in the form of a horseshoe. Then 

 the branchiae originate, and the first two pairs of arms and the 

 organs of hearing. All these organs are formed chiefly at the ex- 

 pense of the inner lamella, the outer lamella serving them only as a 

 protective envelope. On the third day of this period, the rudiment 

 of the mantle presents on the dorsal side a partial thickening of the 

 outer lamella, corresponding to the point which will subsequently 

 serve for the secretion of the os Sepice. 



At this period the embryo is constricted in its middle by a line of 

 demarcation which divides it into two parts : the lower one is the 

 foetus properly so called ; the upper one includes the arms and the 

 vitelline vesicle. The two lamellae of this vesicle separate from each 

 other, although still united by very fine fibres, which are probably 

 prolongations of the cells of the inner lamella. 



On the fourth day the anal tubercle and the rudiment of the 

 siphon (infundibulum) make their appearance. This last is formed 

 of two distinct bands, inclined 45° towards the equator and diverging 

 from above downwards. These bands are thickenings of the inner 

 lamella ; the outer lamella simply covers them, without taking part 

 in their formation. The intestinal canal, the acoustic sacs, the eyes, 

 and the mantle appear with increasing distinctness ; so that towards 

 the end of the second period the foetus already presents the charac- 

 teristic form of ihe Cephalopoda. At this period, also, appear the 

 fins, the third pair of arms, and the nervous and arterial centres. 



On the fifth day the constriction between the foetus and the vitel- 

 line vesicle has become much deeper. The vitelline vesicle begins 

 to perform alternate movements of contraction and expansion of its 

 two lamellae. These movements are due to very delicate fibres, 

 similar to those which M.Mecznikow has described in the amnios of 

 the scorpion. Below the vitelline vesicle the two cylindrical parts 

 which form the true lateral parts of the embryo (that is to say, the 

 cephalic sinuses of M. Kolliker) become prominent. Their outer 

 region is divided into two parts, of which the upper contains the 

 eyes and the ophthalmic ganglia, whilst the lower contains the 

 cartilages and the lateral branches of the vitellus of nutrition. Fur- 

 ther back the mantle and the parts of the body clothed by it are 

 seen. The foetus terminates below in the two projecting fins. On 

 the fifth day, when the number of arms is still only three pairs, 

 none of these appendages is yet furnished with suckers. 



At the end of this second period the two halves of the siphon ap- 

 proach and become united to form aii unpaired organ ; the posterior 

 part of the intestinal canal (independently of the anterior) divides 

 into two cavities, the rectum and the ink-bag, the walls of which are 

 formed at the expense of the outer lamella ; the two pericardia 

 appear at first as solid masses immediately below the branchiae ; the 

 eye becomes surrounded by a layer of pigment, in which, when 



