15 



with what Moldenhauer, His and Gradiuigo have pointed out in animals of the class Mammalia and in Man. This 

 caused me to take the matter into renewed consideration, after my manuscri})t iijjon the 2 emhryos here described had been 

 completed, two years ago. As the drawings show (PI. II, tigs, b, 6), I have not been able to substantiate Howes' and 

 Kiilcenthars statements. In the 2 Lagenorhynchus embryos (26 millira. and .30 millim. long) there is no trace of any 

 indication of protuberances around the auditory oritice. In order to satisfy myself still further as to this circumstance, I 

 examined a Phocæna embryo. 18. .5 millim. long, or three (juarters the size of that described by Kiikenthal; but neither 

 could I here discover the tubercles referred to, surrouniling the external auditory orifice. The external ear opening was tissure-like 

 on both sides, and on the right side slightly concave at the back. It lies on a pertectly flat prominence, which cannot lay 

 claim to the name papilla. 



The contour of the back is almost straight until it joins the curve of the neck (figs. 2, ,5, 6 and 8). On the 

 abdomen, the outline forms a strongly marked convexity in the region of the navel, immediately below which is a 2 millim. 

 long mendn-um. Below this the outhine is formed by a concavity of the narrow cauda, that is turned towards the abdomen. 

 The outline of the side surfaces is curved, with the greatest curve at the level of the navel (fig. 7). 



The small leaf-Jike pectoral fins, that somewhat resemble embryonal paws (cf. figs. .5 — 7), are 4 millim. long, 

 2. .5 millim. broad, and IV2 millim. thick at the root. They spring out from the ventral surface of the body, immediately 

 below the lower jaw, and present a convex outer surface and a concave inner one. On their front margins are some slight 

 indentations (figs. 5 and 6), and the obliquely rising lower edge forms an obtuse angle with the short distal margin. Five 

 thick rays may easily be counted in the pectoral, separated by slight grooves, thinner than the rays, on the upper surface. 

 The hindmost ray or digit forms a projecting obtuse angle between the back margin and the distal margin. The second digit 

 from the front is the longest, and extends into the blunt end of the fin. The surfaces of the pectorals are situated almost 

 in the sagittal planes of the body in such a manner that the outer surface is turned a little upwards or forwards towards the head. 

 On the surface of the abdomen, the arched part lying round the navel is separated from the rest of the body 

 by a shallow, annular furrow (tigs. 3, 5, 6 and 7), and this part difi:'ers from the rest of the body in being of a darker colour, 

 owing to the underlying, blood-filled liver, which is here only covered by the membrana reuniens anterior (Rathké). The 

 circular boundary indicates how far the muscle plates have advanced in the formation of the side walls of the body. 



Immediately below the navel, at the lower edge of the membrana reuniens, the 2 millim. long tuberculum s. 

 membrura genitale projects; it has a fuiTOw (hypospadi) on the under side, and is thickest at the root. There is a very 

 small, oblong prominence here, at the base of the membrum, which I consider to be the commencement of a fold of skin, 

 which is to form the sheath in which the membrum lies during its further developement, and possibly, also, the tirst indication 

 of the mammæ pudendales At little outside the base of the genital member, there is, on each side, another slight 

 prominence (see PI. II, figs. 5, 6 and 7). which is. however, scarcely visible to the naked eye. This slightly conical 

 convexity must be kept distinct from the more oblong prominence at the base of the genital member. In the 30 millim. 

 long embryo, the prominence is a little more pointed, but covers a rather smaller area. "With regard to its interpretation 

 as a temporary external rudiment of the hind limbs, attention is directed to my remarks below. 



On the surface of the abdomen, none but the above-mentioned boundary furrow between the muscle plates of the 

 sides and the membrana reuniens anterior is visible. On the sides and back of the body (tigs. 2, 4 and 5) in the 26 millim. 

 long embryo, is a furrow, concave in front, separating the back from the flanks (dorsal side-furrow). In the 30 millim. long 

 embryo this division between the sides and back is only indicated by a different shade of colour (fig. 8). 



In the examination of the specimens here described, I have naturally given full attention to the much debated 

 question of the rudimentary hind limbs of the Cetacea; but I have not been quite convinced that I have discovered 

 anything, either in the 26 millim. or the 30 millim. long Lagenorhynchus embryo or in the 18-5 millim. long 

 Phocæna embryo that could be interpreted as external rudimentary hind limbs, or rather, perhaps, as 

 rudimentary mammæ. I did not, however, feel at all convinced until a short time ago (in January 1894), irliov I discovered 

 distinct little hind liwhs in a 7 millim. long Phoccena emhri/o, and rudiments 0/ such in a 17 millim. long Phocæna fcetus. 

 After this, I cannot consider otherwise than that the small conical prominences on each side of the membrum genitale, 

 described above, are similar rudiments in L. acutus, these rudiments having, in all probability been more distinctly developed 

 at an earlier stage. 



This explanation has now been most strongly confirmed by examination of serial sections. 



The prominence which Kiikenthal takes to be the rudimentary remains of hind limbs in the Phocæna embryo (25 

 millim. longi. I cannot regard as corresponding to those here described, as the relations are somewhat dift'erent. I shall 

 return to this subject in the description of the small Phocæna embryos. 



The tail, cauda, ends ii a curved cone with the concavity turned towards the ventral surface (PI. II, figs. 1 — 3, 

 5 — 7); near the pointed end of the tail is an indication of a small swelling, which must be explained as the first commencement 

 of the side lobes of the tail-fin. Being now convinced that the rudimentary hind limbs are on the point of disappearing fi'om 



