37 



smaller than the lower, a circumstance which is especially noticeable on the left side. Tliese external prominences are to 

 be interpreted as rudimentary external hind-extremities, their situs and relation to the membrum genitale being in 

 agreement with what we have descriiied in the preceding embryo. These rudiments of liind-limbs are unduobtedly 

 in a state of retrogression in this embryo, and must therefore be regarded as embryonic, disappearing 

 rudiments. 



It should be remarked, that where as these rudiments of extremities at this stage are on the point 

 of disappearing, the Cetacean characters have begun to develope, a few being already very prominent. 



The tail, cauda, is relatively much smaller in this embryo than in the preceding one, the length from the 

 hind limbs to the point of the tail being .5 millim. It has now assumed a more conical form, and the ventro-dorsal diameter 

 is greater than the transverse. It is bent ventrally, and points in a cephalic direction. 



General remarks. It would have been desirable to have had examples of intermediate stages of developement 

 between this 17 millim. long embryo and the preceding one, for this one comes very close to the succeeding 18 millim. long 

 embryo. The usual form of mammal embryo is naturally found here, but there is also an admixture of special Cetacean 

 characters. For instance, the peculiar shape of the external nostrils, differing as it does from that in the ordinary Mammal 

 embryos must be regarded as Odontocete; the distance between the nostrils and the cleft of the mouth is relatively great; 

 and the tapering of the body towards the tail, which continues the trunk without any sharp division, and the absence of 

 hind extremities characterize the more ordinary Cetacean. The vanishing remnants of hind extremities are among the most 

 characteristic conditions of this stage of developement. 



III. The IS millim. long Phocæna embryo, taken on the 19th Sept. 1888 (PL IV, figs. 17—21) is rather 

 more fully developed than the preceding one. The foetal flexures are still, on the whole, strongly marked, especially those 

 of the crown of the head, the neck and the tail (PI. TV, fig. 18), although less so than in the 17 millim. long fætus. The 

 head, measured from the crown to the lower jaw is contained 2V2 times in the total length of the body. Compared with 

 the 26 millim. long Lagenorhynchus acutus embryo, the little Phocæna fætus gives the impression of being somewhat 

 earlier and less developed. Compared with the 2.5 millim. long Phocæna embryo figured by Prof. Kiikenthal, it shows, 

 in addition to its inferior length (18 millim.), a more embryonic character in its external conditions of form. 



The head, although much bent towards the ventral surface, pernuLs a view of the orifice of the mouth itig. 17). 

 The cleft of the mouth produced backwards intersects the axis of the back at an angle of about 40", and the bent conical 

 tail forms almost a right angle with the long axis of the back. The distance between the orifice of the mouth and the 

 nostrils is relatively smaller than in the 26 millim. long Lagenorhynchus embryo; above the nostrils there is a distinct 

 frontal furrow ifigs. 17, 18 and 19); moreover the head is of a spheroid shape, the facial region occupying only a very small 

 and but slightly projecting part, although more distinct and better defined than in the 17 millim. long foetus. The eyes lie 

 proportionately high up towards the cranium, dorsad of the angle of the mouth. The relatively large, open eyes 'I'u x 1 

 millim.) with the eyelids still only in their commencement, exhibit a blue-pigmented iris. 



The external nasal apertures (fig. 19) exhibit no very great difference from those in the preceding specimen, 

 appearing, as they do, as 2 small clefts, anteriorly concave, which are in contact with one another at their mesial ends, 

 where a shallow groeve is observable along the median line. The distance in a straight line of the nasal apertures from the 

 cleft of the mouth is 2V4 millim. This Phocæna embryo is in conformity with the 26 millim. long Lagenorhynchus foetus in the 

 manner of the commencement of the external nostrils, and also in this respect follows naturally the two earlier Phocæna 

 embryos described above, namely, that the external nasal apertures begin, exactly as in other mammals, as two separate 

 openings which are fused secondarily into one transverse fissure in the Delphinidæ. It is interesting to note that the primary 

 opening is already transverse and not longitudinal. In the little Phocæna fætus described by Kiikenthal. the 

 fusion had already commenced. 



The external ear aperture is situated 2'/2 millim. below and behind the angle of the mouth and only a little 

 aljove the production of the inferior margin of the lower jaw (fig. IS). The opening appears like a small V2 millim. obliquely situated 

 (from above downwards, from the back forwards, when the fætus is imagined in a perpendicular position) elongated oval 

 fissure, overhung dorsally by a low limbus (see figs. 18 and 21), so that the whole measures about I millim. in length. On 

 the other hand there is no prominence or papilla to be seen, on which the ear aperture lies, nor could I 

 distinguish node-like prominences surrounding it. as described by other naturalists. The real entrance to the 

 external auditory passage is thus situated in the dorsal region, or the region nearest the head, in a little depression. There 

 is perfect conformity between the right and left sides.') 



The cleft of the mouth is 4 millim. broad, and turns downwards and forwards. The prolongation of the fissure 



') I cannot substantiate Kiikenthul's ileclanition (lit. .58. p. o.^2) as to the dissymmetry of the ear, or the presence of the "sechs Hiigel". 

 On the other hand. I would say nothing against the general dissymmetry in Cetacea, easily substantiated by all. 



