14 



embryo of Lagenorliynchus acutus has commenced a retrograde-degenerative developement, supervening soon after the medullary 

 tube witli the brain vesicles and the more important organs have been formed. It is also somewhat lemarkable that a foetus 

 so small, and which could hardly have been more than 4 weeks old. had it Ik'ch normal, should be found coexistent with 

 line 100 millim. in length. 



II. A far more advanced condition of developement than the foregoing is shown in the 2 6 millim. and the 

 30 millim. long Lagenorhynchus embryos, which resemble in a remarkable degree embryos of other orders oi mammals 

 (Tafel n, figs. 1 — 8). Eschricht delineates a Beluga embryo about 30 millim. in length, and Klikenthal describes an embryo 

 Porpoise 25 millim. long, both of which are in about the same stage of developement as the 26 millim. long embryo here 

 described. The measurements give the distance in a straight line between the uppermost end of the head. i. e. the cranium, 

 and the convexity of the curve of the tail. 



Tiie embryos now under consideration show marked fietal flexures (Tafel II. Fig. 1 and 2. b and 6). with 

 prominent head curvature and less marked neck curvature. The flexure at the back of the neck is still less conspicuous in 

 the 30 millim. long embryo (fig. 8). The tail curve in the 26 millim. long embryo (Tafel II, figs, b and 6) begins almost 

 on a level with the lower edge of the attachment of the umbilical cord, and forms a regular semi-circle; the end of the tail 

 is bent in against the ventral surface. No spiral twists in the direction of the long axis are to be discovered. 



The head is divided from the rest of the body by the constriction of the neck; but this constriction, though deep 

 in front, on account of the ventrally directed jaw region, is very slightly marked at the side and back, though still sufficiently 

 to attract notice. Tiie form of the head is an irregular spheroid with a short and rounded facial part separated from the 

 arched forehead — tlie cranium, by a slight, transversal furrow lying behind the nasal aperture (figs. 1, 2, .5 and 7). This 

 furrow I have named the frontal furrow. The length of the head, measured from the front surface of the upper lip to the 

 curve of the back of the neck, is 11 millim.. thus being between '3 and 'i. of the whole length of the body. 



The nasal aperture in the 26 millim. long fcetus is a transverse fissure I'/a millim. in width, with somewhat 

 raised margins (figs. 3 and 7). On examining it with a magnifying glass, this transverse fissure shows itself to be divided, 

 by a broad wall lying almost close to the surface, into 2 smaller fissures of which the left one is the shorter. These external 

 "nostrils" are slightly concave below, especially the right one, which is the larger. The raised edges of skin are formed of 

 several small, coherent protuberances, of which those on the right side are the larger. On the 30 millim. long Lagenorhynchus 

 embryo (fig. 8) the same thing may be observed, only that there the partition wall lies deeper, while the transverse fissure 

 is continued a little to the side in the shape of a shallow skin furrow. It is thus unquestionable from the above, that the 

 nostrils in the Delphinidæ are double in original construction, and that their fusion is secondary. — The distance between 

 the nasal aperture and the oral fissure is relatively great, viz, 3 millim., an undoubted mark of the Cetacean type. 



The cleft of the mouth is 5V2 millim. broad, measured in a straight line between the corners of the mouth, 

 and turns forwards and downwards (ventro-caudal ; see figs. 5 — 7'. If we imagine it continued backwards, it would intersect the 

 concavity above the flexure of the neck, as this continuation would form an acute angle with the long axis of the body. The 

 side parts of the cleft of the moutli iiave a somewhat sigmoid curvature. The large, arched upper lip or upper jaw projects 

 more than the lower jaw, which is separated by a narrow fissure from the ventral surface of the fore part of the body, 

 A hollow is visible on that part of the lower jaw answering to the mentum (fig, 7), Within the slightly open mouth cleft 

 in the 30 millim. long embryo, the tip of the tongue is visible (fig. 8). No indication of any hair formation is apparent on 

 examining the upper jaw with a magnifying glass. 



The eye is situated above and to the side of the angle of the mouth. The ocular fissure is 1.5 millim. in length, 

 is open, and forms an irregular oval, bounded by the sunken clefts of the, as yet, mere indications of eyelids. The long axis 

 of the oval of the open eye forms an acute angle with the backward production of the angle of the mouth. 



The ear aperture lies about 3 millim. behind and a little below the angle of the mouth, almost midway between 

 the jiiouth cleft and the Ijend of the neck, the continuation of the cleft between the lower jaw and the liody being below. 

 The ear aperture forms a short, oblique cleft, 0.5 millim. in length, with almost imperceptibly raised margins. On using a 

 lens of a higher power, the oval or round opening of the auditoi'y passage in the 2 Lagenorhynchus embryos here treated 

 of, is seen to be situated in a little oblong, almost fissure-like cavity, which may presumably be considered as homologous 

 with the fossa angular is aur is; but no surrounding ridge or other elevation is perceptible. 



The external ear of the Cetacea is a subject tliat has of late received a considerable amount of attention from 

 investigators. Besides being mentioned by Eschricht i4Ii, a rudimentary pinna has been described by G. B. Howes, 

 from a Phocæna embi-yo, and even from full-grown Porpoises. Kiikenthal (58) has lately described very exactly the 

 developement of the external ear in Whales, having found in his 25 millim. long Phocoena embryo, that the external cavity, 

 the fossa angularis auris, was surrounded by a )-aised wall with small iirominences, of whicli he could distinguish 6, corresponding 



