18 



Below and behind the cleft of the eye is found tlie excessively small ear aperture, which may be recognised as 

 a small cavity with a tiny hole in it. 



The under surface of the lower jaw makes a more and more ol)tuse angle with the surface of the abdomen, as the 

 eniljryo increases in size whicii is an indication that the facial region, togetlier with the jaw and throat increase relatively more. 



Tlie pectoral fins licgin to assume more their characteristic form. The convex ventral margin and the projecting 

 angle on the dorsal margin are also peculiar. The digit-rays show distinctly through. 



Tiie rudimentary dorsal fin imparts to the formerly straight outline of tlie l)ack a convexity wliieh is opposite to 

 the umbilicus. Though scarcely projecting in the 73 and 78 millim. long embryos, it measures from 1 to 2 millim. in the 

 embryos over 90 millim. in length. There is also visible a little comb-like elevation in the medial line, extending backwards 

 right on to the tail. — The insertion of the umbilical cord lies near the middle of the body. In the embryo that is 

 over 90 millim. in length, there is still a broad furrow in front and behind this insertion, the boundary between the rapidly 

 approximating muscle-plates of the sides of the body. 



'Phe sharjily curved membrum genitale conceals the entrance to the urogenital canal. At the base of the 

 membrum indications of the mammæ pudendales are observable in two small prominences, in each of which, by the aid 

 of a magnifying glass, a minute hole is seen. The very small anal opening is situated not far behind the genitalia (iig. 14). 



The tail has lost much of its conico-cylindrical shape, and has become flattened at the sides, the dorso-ventral 

 diameter having, relatively, increased considerably. The lobes of the tail are large, triangular pieces. The whole cauda has 

 straightened itself out, still more than in the earlier stages, in the direction of the long axis of the body, with a slight 

 inclination to tlie right. 



A considerable change of colour may be noticed in all the fætuses of this stadium. In the region of the back 

 of the neck, there is a broad, dark band of colour. — the "neck-band'", extending down to the region behind the eyes. In 

 front of this dark band, the skin of the head is of a very slightly darker shade, while behind it, on the back as far as the 

 dorsal fin, a darker skin colour is clearly visible (cf PI. II, fig. 12). This blackisli grey colouring of the skin may also be 

 traced some way down over the sides. Tlie dark variation in colour is inconspicuous on the smaller embryos, stronger and 

 more distinct on the larger ones. No change, however, has taken place in the eye region, the snout, the ventral surface or 

 the tail, in those embryos that are less than 10 centim. in length. The natural skin colour of embryos just taken from the 

 uterus is always pink, with a little grey in it, the pigment or dark skin-colour appearing in the shape of tinges of colour 

 from grey to blue-black. This pink ground-colour with the greyish tinge in it is owing to the quantity of blood in and 

 succulence of the tissues, while the darker colour is caused, as is well known, by the pigment deposit in the skin. By 

 hardening in alcohol, the reddish colour is changed to white or greyish red. while the dark colour remains almost unchanged, 

 always presupposing that the embryo has been preserved in a fresh condition and not allowed to lie for some time, so that 

 cadaveric changes have set in. 



Y. The (jcneric characters begin to appear in fætuses of Lagenorhynchus acutus, Gray, that are from 10 

 to 20 centim. in length (PL III, figs. 1 and 2). The characteristic features of fcetuses of this size (which is from Vo to '/s 

 of the size of the full-grown foetus) may be shortly recapitulated as follows: — 



The snout points almost straight forwards, so that the foremost part of the head is formed by 

 the region extending from the external nasal apertures to the apex of the upper jaw; the pectoral fins 

 have the semilunar shape peculiar to Dolphins, and, while the fætal tailflexion has disappeared, the caudal 

 segment of the body begins to assume a ventro-lateral curve; to these is added a dorsal fin several 

 millimetres in height, while, between the large, triangular lobes of the tail-fin, there is a little cleft, 

 and their posterior edges take the shape of an S. Sexual difference is here very distinct. 



It may be remarked, with regard to the relation between the body and its various divisions, that the head amounts 

 to about 'A the lengtli of the body, the pectorals to from V4.V to Vs.e, wlnle the navel insertion lies at about the middle of 

 the ventral surface of the body, though frequently a trifle further back. 



The head. In proportion as the noss takes a more forward and upward direction, the angle between the lower 

 jaw and the surface of the abdomen disappears (fig. 2). The arched upper jaw becomes more pointed anteriorly, and the 

 slight groove beginning a little above the angle of the mouth, and ascending obliquely on the nose towards the median line, 

 becomes more j)rominent, forming tlie break peculiar to the Delphinus family, viz, the "beak". On the upper lip may be 

 counted the commencement of 6 or 7 hairs; the upper lip is in the form of a gentle sigmoid curve, which jiroduces a 

 convexity downwards, somewhat behind the apex. The lower lip is a little everted, the retreating upper lip overhanging it 

 at the angle of the mouth (PI. Ill, fig. 2). 



