35 



the insertion of the innl>ilieal eorcl on tlie ventral siirfaee, a triangular, roumleil, genital eminence projects, — tlie tuberculum 

 genitale (tig. 6, t g). A little dnrsad of the genital eminence at the root of the tail, distinct external rudiments of hind 

 limbs project from the sivie surfaces of the body. The distance from tlie root of the tail to the point is about 4 niillim. 



The extremities. The fore-limbs, IV3 millim. long and 1 millim. broad (figs. 6 and 7, f. Ij, are of a rounded, 

 paddle-like shape, the base being a little narrower than the flat and bud-like free end. There is a slight indication of 

 division into two segments. The long axis of the fore-extremities runs obliquely in a caudal and ventral direction. There 

 is no external indication of finger rays. 



The hind-extremities (fig. 6 and 7. h. e. and fig. 12, h e) are O.lh millim. long and 0.60 nnllim. broad, and are 

 like oval leaves in shape, narrower at the base. The free, rounded margin is turned ventrally and a little laterally outwards, 

 while the attached base is situated on the flanks at the junction of the trunk with the tail, ventrad of the dorsal side-groove. The 

 proximal end lies on a level with the cephalic margin of the genital eminence, and therefore far below or caudad of the umbilicus. 

 The long axis of the extremities runs in an obli(|uely caudal and ventral direction (fig. 12, h. e). The position of these small 

 hind-extremities thus agrees completely with what we know of the embryos of other mammals. They are, however, scarcely 

 more than half the size of the fore-extremities, a point of difference between these f "d other mammal end>ryos, in which the 

 hind-extremities, though developed somewhat later, are not far behind the fore-limbs in i)oint of size, even in the early stages 

 of developement. On making serial sections through the right hind-extremity, it proved, on microscopical examination, to 

 consist of undifferentiated mesoderm tissue without sharp separation from the epidermis: in some places small vessels 

 were observable. — We thus have before us an embryonal elementary form, which is at first small and 

 slender, showing no tendency to powerful developement. Compared with the embryos of (piadruped mammals, there 

 is already a great difference in the first commencement of the organ in this respect. When the rudimentary hind-extremities 

 are compared with the same embryo's relatively large, powerful tail, we also find a striking contrast in the developemental 

 tendency of these organs, the hind-extremities, which are destined to disappear, being, from their earliest appearance, weak 

 and small, while the tail, which afterwards becomes the animal's chief organ of locomotion, and, as such, displays a high 

 degree of developement and functional adaptation, is found, even in the earliest stages, in the front rank of the external 

 of the body, on account organs of its powerful elementary character and rapid growth. 



The umbilical cord is IV3 millim. in diameter, and very short, between 2 and 3 millim. in length, its veins 

 soon being absorbed into the ftetal membranes. The amnion is attached about l^/a millim. outside the abdominal insertion 

 of the umbilical cord. 



From the cephalic margin of the navel, or in other words, the margin nearest the chest region, projects a very 

 narrow cylindrical process, about 2 millim. in length, with a slightly club-shaped, expanded free end (fig. 12, v. u.). This I 

 take to be the vesicula umbilicalis, which is connected with the gut by a short ductus omphalomesentericus. 



General remarks. This smallest Phocæna embryo, which may be said to measure about 14 milHm. when the 

 distances between the fcetal flexures are added together, presents all the characters in their general form, and as we are 

 accustomed to find them in other mammal embryos in a corresponding stage of developement. One peculiarity which may 

 be noted is that it is very small and slender, a human embryo of about the same degree of developement being l)oth longer 

 and considerably thicker. The size of the head, moreover, seems to he small in proportion to the body, while the tail is 

 unusually long. As already indicated, the rudiments of the hin<l-extremities are very small in proportion to the fore-limbs, 

 when compared with the embryos of four-footed Mammals; and though the external form of the limbs in this Phocæna fætus 

 does not, on the whole, seem to differ from the ordinary embryonal limb form, I consider the paddle-like shape of the hind- 

 extremities to be somewhat characteristic. The transversally situated olfactory depression also seems to me to be characteristic, 

 and perhaps already denotes an Odontocete character. At any rate, the shape of the nose here dift'ers distinctly from the 

 shape of that organ in a somewhat larger fætus of Phoca griinlandica (17 millim. measured from the flexure of the head 

 to the tail), where the nasal cavity has a vertical direction, while in the Phocæna the olfactory groove opens transversally 

 into the more vertically running lachrymal groove. — The powerfully developed tail is a peculiarity which was also conspicuous 

 in the 8 millim. long Lagenorliynclius acutus embryo. We may see in this an expression of the apparently general law 

 in the developement of the organs, that an organ which plajs a jy c(>ns[»icii<ms part in the iii(lei»eiulent existence of the 

 animal (of the species, the genus, the family, etc), appears early, and even in its first embryonal developement makes a rapid 

 growth, and assumes a more or less prominent position (e. g. the eye in birds, the fore-limb in the bat, the brain in man >. 



II. Tlie Phocæna embryo, that is scarcely 17 millim. in length, measured from the flexure of the head to the 

 bend of the tail, resemliles greatly the succeeding one. showing, however, in several particulars, a far less advanced condition 

 of developement, although the difl'erence in length — about 1 millim. — is very slight (PI. IV, figs. 2. 3, 13—16). 



The fætal flexures are less marked than in the preceding specimen, but more so than in the 18 millim. long 

 embryo (PI. R', figs. 17 and 18). The flexure of the neck (fig. 13, m forms an angle of 130" or 140" with the long axis of the body. 



