36 



and the lic-id Hc^xure about tlic same, the cruwn of the head being the highest point of the body, if the fietus Ije imagined 

 in a vertical position. Tiie tail flexure is much less than in tlie previous stadium, and the tail itself is relatively uiuch 

 shorter. The umbilical cord is attached at the transition from the middle to the hindmost third of the body. 



The head. Although the cranium certainly forms the main part of the head, the face region has already made 

 <lecided advance. A shallow furrow is formed in front, — the frontnl furrow (tig. 13. ft'), — dividing the fore part of 

 the cranium trom the face, this furrow being traceable almost to the upper border of the eye-region. The surface of the 

 cranium exhibits slight i)rominences corresiionding to the brain vesicles, one slight swelling being found on the frontal portion 

 of the head corres))onding to the hemispheres (tig. 13, he), another on the crown for the mesencephalon (tig. 13, m). in 

 front of this the th alamencephalon (fig. 13, the) and a depression between the crown of the head and the ftexure of 

 the neck, answering to the 4th ventricle itig. l.i f..rh). Through the integuments, the fovea rhomboidalis is distinctly 

 (liscernil)le in outlines 'tig. 1.5) which show excedingly large lateral prominences, and also the elongated medulla oldongata 

 and the spinal cord below the flexure of the neck. 



Seen in i)roflle (flgs. 13 and 16). the facial region constitutes a very small part of the head. It is however, 

 noticeable that the upper jaw is very predominant. — an indubitalile Cetacean character which has begun to assert itself 



The eye is situated on a level with the nasal orifices. The length from the inner to the outer angle of the 

 eye measures IV2 millim., the height being much less; the eyelids have begun to appear in the shape of little folds of skin, 

 and the pigmented iris is distinct. Within this, several fine rings are visible, while the centre is occupied by a white spot 

 — the pupil (figs. 13 and 16). 



The nostrils (fig. 14). each about 1 millim. wide, are 2 sigmoid, transverse fissures, connected in the median 

 line by a shallow groove. In this point they already exhibit a marked Odontocete character. The distance between them is 

 relatively large, but the shallow connecting groove already denotes the incipient external fusion, as it is found in more 

 advanced embryos. While the distance between the nostrils and the frontal furrow is very small, we see, even at this stage 

 of developement. the rapid growth of the upper jaw, which projects and separates the nasal orifice from the mouth orifice. 



The orifice of the mouth is turned towards the chest, and cannot be fully seen, only a small part of the 

 corners of the mouth being visible at the sides (see figs. 13 and 16). It is about 4V2 millim. broad. The lower jaw is but 

 little developed, and is overhung considerably (about 2 millim.) by the already greatly projecting upper jaw. 



The external ear aperture, from 0.7.5 to 1 millim. in diameter, is situated 1 millim. above the fore-limb and 



2 millim. behind the corner of the mouth, obliquely below and behind the eye. The aperture is funnel-shaped, oval, sharply 

 defined in front, but more open behind. The small auditory passage is situated in the lower part of the oval depression 

 (figs. 13 and 16). The whole of the external ear aperture is surrounded by an exceedingly slightly-marked, annular eminence 

 which has a smooth and even surface, without any trace of small prominences or depressions which might denote an incipient 

 external auricula. 



Just below the head, at the constriction of the neck, the fore-extremities protrude (Fig. 13. 14, 16). They are 



3 millim. long, and lie flat against the side of the body; they consist of a narrow proximal part, scarcely 2 millim. broad, 

 and a broader distal part, 2V2 millim. in width, the latter being of a rounded, paddle-like shape. In the broader distal end, 

 5 divergent finger-rays are distinctly discernible. 



On the back, the spinal cord is distinctly seen through the integuments (fig. 15), beginning immediately below 

 the flexure of the neck, where it exhibits a narrow and as it were constricted part, which, however, expands just below into 

 a slight but definite intumescence, (intumescentia cervicalis) on a level with the fore-limbs (fig. 15, ic). The spinal cord 

 is visible superficially right down to the tail flexure. Before it reaches that point, there is an indication of a faintly defined 

 enlargement (lumbar intumescence, fig. 15, i 1.). 



On the surface of the back on each side of the spinal cord visible through the integuments, is a "lateral back- 

 furrow'' running parallel with the median line. This furrow extends from the flexure of the neck down to the tail. On the 

 sides of the body, and especially at the level of the umbilicus, a lateral side-furrow is distinctly apparent (fig. 13 and 16). 

 Nearer the ventral surface, a "ventral side-furrow'" denotes the boundary between the side plates and the membrana 

 reuniens anterior. This boundary furrow extends from the neck down to the region immediately above the membrum 

 genitale (fig. 14). The ventral surface is very convex, as is usual at this stage of developement, and its lower portion is 

 occupied by the origin of the funiculus uml)ilicalis. 



The membrum genitale is about 2 millim. long and very projecting (fig. 13. g). It forms a slightly curved 

 cylindro-conical prominence, the broad base of which occupies the space between the concavity of the tail flexure, and the 

 lower boundary of the membrana reuniens anterior. On the under side is the legitimate groove (hypospadia), which passes 

 jjosteriorly into the orifice of the sinus urogenitalis. 



A little dorsad but at the base of the membrum genitale, are visil)le on each side, two small tubercles, which 

 are connected at their basal origin, and project about Va millim. (Fig. 13 h e. figs. 14 and 16.). The upper tubercle is 



