16 



the surface of tlie body, and that it is only after tlieir disapiifarance that the very lonjr tail shows the commencement of the 

 formation of flukes, this may with certainty be taken as an ontogenetic proof of the secondary appearance of tail flukes as 

 a more lately acquired form. How far this fluke formation may be deduced from a primary lateral widening of almost the 

 whole free portion of the tail (Ktikenthal; 58, II Theil, p. 263), as regards Lagenorhynchus acutus, I am not prepared to say. 



Recapitulation. In the general impression gained of the 2(i and 30 millim. long embryos now before us (figs. 

 1 — 8), their mammal-like qualities are those which stand out most conspicuously, their likeness to small embryos from other 

 orders of that class being quite remarkable. On closer consideration, however, it will at once be observed that the special 

 Cetacean type has even nt this stage of developement set its peculiar stamp upon these embryos. The relatively great 

 distance between the nasal and oral apertures, the external nasal apertures fused into one transversely-situated cleft (indicating 

 the Odontocete character), the short neck and the long tail, the absence of external ears and external hind extremities, while 

 the fore extremities are already well developed, mark the end)ryo as Cetacean. 



The characters of the class and order are thus, even at this stadium, intermingled, a fact which is 

 interesting to note, as it is usual, in the earlier stages of developement, to see only general or phylogenetic characters. 



III. A more advanced condition of developement in the Cetacean type is shown in the 45 millim. long emljryo 

 (PI. II, iigs. 9, 10 and 11). Among the fætal flexures, that of the neck is almost obliterated, while that of the crown of 

 the head, the principal cephalic flexure, and that of the tail are still distinct. The snout portion of the head forms a right 

 angle with the long axis of the body, and the constriction of the neck is distinct only in front, there being but a slight 

 indication of it at the sides. 



The round head with short, pointed nose, the oliliquely-situated, closed eye fissures, the inward-turned pectoral fins 

 beginning to assume a semilunar shape, and the still conical tail, where the first trace of the tail-fin at the point, gives it 

 a bud-like appearance, are the characteristic features of this nearly 5 centim. long embryo, which has attained to about ^2o 

 of the size of the newly-born animal. The umbilical cord is attached beyond the middle of the body. 



With regard to the details concerning this embryo, the following is to be noticed: — 



The head has an almost spherical cranium and a short, conical facial region. Its height from the neck to the 

 crown of the head measures about Vi of the length of the body, its natural length, i. e. from the end of the nose to the 

 bend of the back of the neck, measuring about Vs of the length of the body. The diameter from the mouth to the back of 

 the neck forms a right angle with the long axis of the body, and measures 18 millim. The slightly curved external nasal 

 aperture is a 4 millim. broad, transverse fissure, and may be said to form the boundary between the snout and the forehead 

 which arches upwards and disappears into the round cranium. The external nasal fissure has rather full lips, and shows an 

 indentation in the middle: the right side region is a little larger than the left. — The facial region or snout seen in profile, 

 resembles a short, curved bird's beak (fig. 10). The upper jaw is arched, the lower more straightly projecting and forming, 

 with its lower surface, an obtuse angle with the ventral surface of the body. The ti.5 millim. broad moutli fissure measures 

 4.5 millim. in length from the point of the jaws to the angle of the mouth, and runs straight in perpendicularly to the long 

 axis of the body. On each side of the upper lip, a little above the mouth fissure is a slightly raised line with from 6 to 8 

 wart-like protuberances, which are indications of the incipient fu'tal hairs. The edge of the lower jaw is narrow in front, 

 broader lieyond, and inclined a little downwards; the edge of the upper jaw is also narrow in front, and inclined a little 

 upwards beyond. The edges of both lips form a conical point in front. 



The I'/'-i millim. long eye cleft is situated at a distance of 4 millim. from the mouth fissure, with which it forms 

 an acute angle, if the uKuith cleft be imagined produced as far as to the eye. In a fronf aspect, the eye region forms a 

 slight convexity. A little below and behind the eye, the external ear appears in the shape of a spot somewhat different 

 in colour from the rest of the skin, and whose greyish yellow hue cannot easily 1)0 distinguished without the aid of a 

 magnifying-glass: on the right side it forms a small, very slightly projecting proniinence. 



The depression between the crown of the head and the back of the neck, which was very distinct in the previous 

 stadium, is here effaced. 



The contour of the Itack is almost a straight line down to the tail, which has only a slight, ventrad curve, and 

 (loints downwards and to the side (PI. II, figs. 9 — 11). The ventral surface is not nearly so arched as in the 26 millim. 

 long end^ryo, and the side surfaces from the head down towards the tail, present only a slight convexity (fig. 11). The 

 trunk is thickest between the navel and the anterior extremities. Beyond the navel, the cylindrical body rapidly decreases 

 in thickness. 



The fore-extremities have already, to a great extent, assumed the shape of those of the Delphinidæ. They 

 lie with their pointed lobes inclined inwards towards the median line. Measured in a straight line along their front edge, i. e. 



