38 



backwards passes between tbe eye and the car aperture. The ui>p('r lip. above the fissure, projects beyond tlie rnouth and 

 the lower jaw. The latter has a triangular depression below tlie mouth cleft (Fig. 19). 



The constriction of the neck, which seems to be characteristic of Dolphin embryos in their earlier stages, 

 thus recalling small eml)ryos of terrestrial mammals, appears here in the same manner as in the 26 millim. long Lagenorhynchus 

 embryo. At the back of the neck, there is a depression corresponding to fovea rhomboid alis. The fore-limbs project 

 just below the constriction of the neck. 



The fore-extremities (fig. 19) have almost the same shape as those in terrestrial Mammal embryos of an early stage 

 (if developement, namely a short, broad arm, with the distal portion broadly rounded and paddle-like: the medial surface is 

 turned towards the chest. On the distal, paddle-like p.art, b short, divergent finger rays are seen through the skin. 



The dorso-ventral diameter of the trunk is longer than the transverse diameter. The edges of the back are 

 comparatively straight, while the ventral surface and sides form convex lines of contour. The embryo's ventral surface, as 

 in the preceding example, is a very convex part, with its boundaries clearly defined by a ventral side furrow (fig. 18). The 

 convex abdominal region is covered by the membrana reuniens, while the ventral side furrow indicates the boundaries of the 

 advancing muscle plates. There are no other furrows to be seen on the sides of the body, except a slightly marked side 

 furrow, dorsad of the fore-extremities (fig. 18), and a back furrow parallel to the median line. There are no transverse 

 furrows or prominences on the side region on a level with the imibilicus and the genital organs. 



The navel, situated in the lower part of the ventral area, measures 3 millim. in a cephalo-caudal direction. In 

 the hardening process, its sides have been compressed. 



A little below the ventral area which is covered by the membrana reuniens, is situated the membrum genitale 

 (figs. 17. 18 and 20) which is thickest at the base and tapers forwards. It projects below the navel and forms a concavity 

 downwards. On the under side is a groove (fætal hypospadia). 



At the base of the membrum, a little below its root and attached to it, the skin is seen to be slightly raised. 

 Somewhat beyond, laterad of the base of the membrum a small skin prominence is observable (fig. 20). This I explain as 

 the finally disappearing rudiment of an external hind-limb. On first noticing this, I was inclined to take it to be 

 the first commencement of mammæ, though I could not affirm it with certainty until I could convince myself, by serial sections 

 through that region, of the actual conditions. When, however we compare these external conditions of form with those in 

 the 2 earlier Phocæna embryos and the Lagenorhynchus embryo, the explanation is clear. The small prominence which is 

 situated laterally as regards the base of the membrum, has exactly the same relative position as the soon-to-disappear, 

 external rudiment of a hind-limb in the 17 millim. long embryo, while the prominence within, close to the base of the genital 

 member is the incipient fold of skin, which becomes the labia majora resp. the skin sheath for the penis. Upon or by these 

 begin to appear somewhat later the mammæ pudendales. These, in my opinion, are a formation which first appears 

 when the rudiments of the hind extremities have almost disappeared. 



The transition from the truncus to the cauda, though not abrupt, is rapid. The cauda is cylindro-conical, and 

 measures 3V2 millim. in length from the anus to the apex. It forms almost a right angle with the long axis of the body. 



General remarks. This 18 millim. long Phocæna embryo, which differs only slightly from the preceding 17 

 millim. long embryo, is, however, more developed in its specific Cetacean characters, while it still recalls, more than the 26 

 millim. long Lagenorhynchus fætus, embryos of terrestrial Mammals in its general form, the constriction of the neck, the 

 general paddle-like shape of tbe fore-extremities, and the ordinary fretal flexures. As special Cetacean characters may be 

 noticed the position of the external nostrils high up on the face, and their transverse cleft form, which is a Dolphin 

 characteristic, the simple external ear, the absence of hind-extremities, and the relatively long cauda. which passes without 

 any decided bountlary into the trunk. The early developement of the membrum genitale is also peculiar to the Cetacea, 

 according to the exi>erience I have up to the present acquired. 



From the above description ot the small Phocæna embryos, together with the facts learned from the Lagenorhynchus 

 embryos, we may afi'irm that rii(liiiieiitary hind limbs begin to appear in Ilie Delpliinidæ in an oval, rnnded, 

 paddle-like shape at a ver.v early stage of developement, vix. wliile the visceral clefts are distinct. Their 

 position on each side of the ))ody is on e level with the genital tnherele, and corresponds exactly with the 

 place where the external commencement of hind-limhs generally appears in the higher vertebrate animals. 

 From the very beginning they are small, and appear like tiny, thin superficial appendages, scarcely half the size of the fore- 

 hmbs. They are soon left behind in developement, and undergo a retrogressive process, so that their presence as superficial 

 appendages is only temporary. As they disappear, the Cetacean characters appear, and when they have 

 altogether disappeared as superficial appendages, the first commencement of a mammary organ is to be 

 found. In contrast to the temporary superficial hind-limbs, the tail, in the earliest embryonal period, is found as a rapidly 

 developing external organ. The fact that special characters, indicating the direction of the developement. appear so early, 

 ought also to be noted as a property of perhaps general force. 



