* 

 SEAL-EMBRYOS. 9 



Manus. lu an embryo 64 mm. long the five digits are quite visible and 

 distinctly webbed, the webbing extending to almost the tips of the fingers and, at 

 this stage, being quite as well marked as in the foot. At this stage the second digit 

 is slightly longer than the others, the fifth being the smallest. The nails begin to 

 appear when the animal is rather older. Beyond the change in position of the limb 

 described above, the manus retains these characters throughout foetal life. 



Pes. At their first appearance the digits are spread out in a fan-shaped manner. 

 They are sub-equal in length ami united by a web. The outermost digits on both 

 sides soon commence to elongate, so that the tips of the digits are in a line with 

 each other (figs. 5, 6). This increase continues until the first and fifth digits are 

 longer than the intervening toes (fig. 7), a condition which obtains throughout the life 

 of the animal. A web extends between all the toes. Those portions of the web which 

 pass between the first and second digits and between the fourth and fifth are very 

 short, so that their mobility is considerably restricted. The portions of the membrane 

 on each side of the central digit are much longer. This condition is shown in the 

 text (fig. 8), which is drawn from the foot of the largest foetus. 



Accompanying the elongation of the outermost toes there is a considerable amount 

 of flattening and lateral expansion, each toe having a width of 2 5 cm. The flattening 

 becomes more marked towards the extremity where the digit is almost membranous. 

 The nails are terminal and recurved in the earlier stages, but, owing to the elongation 

 and expansion of the fleshy parts of the toes, more particularly the first and fifth, they 

 come to be on the dorsal surface 1 3 cm. from the distal margin. After the nails have 

 once been formed they increase but very slightly in size ; in the specimen here 

 represented the free portion of the nail has only a length of 3 mm. Those on the 

 three central digits are rather longer and are placed somewhat nearer to the extremity. 

 Visceral arclies. As might be expected, these are only visible in the very earliest 

 stages ; the intervening clefts are not perforated (fig. 2). 



In the later stages the subcutaneous tissues become laden with fat, the lobules 

 being bound together by very tough connective tissue. The rapid accumulation of 

 fat in the skin as the intra-uterine life is drawing towards its termination, is in 

 preparation for the young animal's independent life in the frozen waters of the 

 Antarctic seas. 



SKELETAL AND MUSCULAR SYSTEMS. 



I am able to add but little to the accounts already published by Leboucq, Murie, 

 and others, with regard to the anatomy of the skeleton and muscles. There are, 

 however, a few points of interest to which attention may be drawn. 



Skeleton. Skull and Vertebral Column. With the exception of the basis cranii, 

 the process of ossification had not advanced to any great extent even in the oldest 

 foetus. In general shape the skull has the characteristic adult appearance from quite 



VOL. V. C 



