34 



an oval rlrvatioii clon.natcd transversely, wliicli is the anterior primary cerebral vesicle, with indications of a division in the 

 middle, forming the two vesicles of the hemispheres (tig. 11 h. m.i. On tlie ventral surface of tlie iiead — the face — may 

 \>r plainly seen below the elevation of the fore-brain vesicle, the fronto-nasal process (Stirnfortsatz. Kolliker) with a broad 

 depression in the median line, forming on either side a l?.rge. oval prominence (the nasal prominence) with a cleft-like opening, 

 viz. the olfactory jiits tig. 11 Ng.). These are situated at a distance of about l' •.. millim. from one another, and form 

 an oval depression of about ' - millim. in length, situated ti'ansversely in relation to the long axis of the body, and contracting 

 towards its lateral end into a narrow groove (tig. 11) which opens into the boundary groove of the maxillaxT process (the 

 lachrymid groove, tig. H, b). The olfactory pits are bounded by raised lips. The narrow groove running laterally from the 

 olfactory depression may be explained as the "olfactory groove'" iNasenfurche, Krdliker) It does not. however, open directly 

 into the orihce of the mouth, but into the lachrymal groove running from the mouth cleft u)) towards the eye, which defines 

 the maxillary process mesially. 



\\'e thus notice from the very first a transversely situated olfactory depi'ession. by which we can 

 distinguish between the "superior" and "inferior" nasal processes, corresponding to the "external" (lateral) and "internal" 

 (mesial^ nasal processes in man and other mannnals. 



Below the nasal orifices, the median region is occui)ied by the broad mouth cleft which expands upwards and 

 downwards in the median line. The cavity of the mouth seems to form a conical depression behind in the direction of the 

 back of the neck, where it closes. A slighter depression in the floor is probably the opening to the fore-gut. The orifice 

 of the mouth is bounded on each side laterally by the but slightly prominent "maxillary process" (figs. 6, 7 and 10 m. x.), 

 above and laterally by the lower margins of the olfactory depressions. Thus in this stage of developement. in opposition to 

 the subsequent ones, there is only a short distance between the olfactory depression and the orifice of the mouth. The latter 

 is bounded below by the first visceral arch, or mandibiila, which here lies against the chest region (figs. 10, 11, m b). 



On viewing the side surfaces of the head (figs. 6, 7 and 10). a shallow depression is seen above the maxillary 

 process, and behind the olfactory depression; in the shallow depression a slight convexity is conspicuous — the cornea (fig. 

 1 1 ,o). The invagination of the lens has probably taken place a short time before, and the eyeball has not yet advanced 

 very far. only a very slight protrusion of the cornea being noticeable. There is. as yet. no trace of pigmentation in the eye. 



In a profile view (figs. 6, 7 and 10). the first visceral or mandibular arch is distinctly seen, with its relatively 

 lu-oad and short "maxillary process". This latter is separated from the eye and the lips of the olfactory depression by a 

 shallow groove, as before mentioned. The mandibular arch is thickened and rounded in its transition from the side to the 

 ventral surface, where it tapers towards the median line. A slight depression indicates the symphisis in the middle (see fig. 11). 



The first visceral cleft has the appearance of a narrow, but distinct groove separating the mandibular from 

 the hyoid arch I'fig. 10). There is no indication of small prominences separated by shallow depressions such as might 

 represent the first rudiments of an external ear i auricula). 



The second visceral or hyoid arch (fig. 10 hy) is shorter and a little narrower than the first visceral arch; its ventral 

 portion lies close in to the chest, and could not therefore be more closely examined in the embryo while uninjured. This 

 arch is bounded caudally by a relatively large, and about V2 millim. broad, rounded depression (fig. 10, fp\ which is triangular in shape 

 with its base towards the 2nd visceral arch, its apex pointing in a caudal direction and all its angles rounded. The dorso- 

 caudally situated portion of this depression is still more hollowed out into a little blind foramen, of which the bottom is 

 distinctly visible. The part of the triangular depression lying nearest to the hyoid arch is the 3rd visceral arch. This 

 depression may also, from its situation and relations, be supposed to be the bend of the throat, sinus cervicalis (s. 

 pi'æcervicalis). It is surrounded by a little wall — limbus (fig. 10. fp). 



The trunk. From the flexure of the neck down to the bend of the tail, the series of protovertebræ may be 

 distinctly traced externally, viewed either from the back or side (figs, fi— 8). About 0.7 millim. below the curve of the 

 throat, the fore-limbs spring out. Ventrad of these, the chest bulges out, and a kind of external boundary between the chest 

 and heart region, and the abdomen proper is formed by a narrow groove running from the upper proximal end of the fore- 

 extremity in a ventral and caudal direction. But the chest region is also divided Ijy a short vertical groove into a more 

 projecting ventral part (cor) and a narrower dorsal part which is continued dorsally (i. e. caudally» below the bend of the throat. 



The sides of the body are moreover separated by a shallow, dorsally-situated furrow running parallel with the 

 curve of the hack. — the "dorsal side-groove" — from the dorsum proper. The shallow "ventral side-groove" starts from 

 the axil, and runs obli(|uely downwards (caudally) and ventrally to the side of and below the funiculus umbilicalis, by which 

 the abdomen proper is separated from the flanks. 



Not far beyond the umbilicus, caudally. the curve of the tail begins, the body here describing an arc of 180" in 

 jiassing into the tail. 



The tail which is conico-cylindrical, gradually tapering towards the point, is bent up against the ventral surface 

 of the body and reaches almost to the face, although the end of the tail has a slight ventral inclination. Immediately below 



