8CHULTE AXD TILXEY, XEURAXJS IN THE DOMESTIC CAT 339 



profundus (3a) between the first and second segments (31, 12) ; the 

 quintus (3) between the second and third (12, IS) ; tlie acoustico-facial 

 (Ii) immediately behind the third. Ventrally the second segment shows 

 signs of subdivision. Barely seen in embrvo Xo. 188, in No. 548 it is 

 marked by a vertical furrow ascending to the attachment of the quintal 

 ganglion and dividing the segment into an oblique cranial portion (12a), 

 extending completely across the neural tube and a Triangular caudal 

 fraction (12h) which forms the bulk of the pontine angle but is confined 

 to the ventral region of the neural wall, being excluded from, the alar 

 portion by the oblique complete third segment. The myelencephalon has 

 seven neuromeres. These contrast sharjjly with the foregoing by their 

 vertical position. The oblique nietencephalic segments are now con- 

 tinuous with the neuromeres of the m}-elencephalon ; that they form a 

 homodynamous series is certainly not supported by the evidence of their 

 development in the cat, in fact the heterogeneity of these elements seems 

 as clearly given by their ontogeny as by the diversity of the peripheral 

 nerves with which they are associated in the adult. 



Embryo of Sixteen Somites. — (Plate XXXA^.) The neural tube is 

 closed and completely separated from the ectoderm except in the region 

 of the fossa rhomboidalis. The flexure at the posterior isthmian sulcus 

 (23) has increased and the nuchal bend is now present. In the hind- 

 brain the region of the second (13) segment projects ventrally and 

 forms the pontine angle. The optic vesicles (1) are still further reduced 

 in size, absolutely as well as relatively, and there is formed both ventral 

 and dorsal to the vesicle a considerable zone which represents the exten- 

 sion of the telencephalon (19) and infundibular region (18) of the 

 preceding embryos. Tlie coalescence between the mammillary region 

 (17) and the thalamencephalon (16) has increased; and they now form 

 a well-marked segment between the midbrain and the derivatives of the 

 optic vesicles (i)." The mesencephalon (15) is triangular and markedly 

 compressed ventrad. The segments- of the hindbrain are less oblique 

 than in preceding embryos. The first (31) is large, the second (12a. 

 12h) forms the prominence of the pontine angle, is ventrally subdivided 

 and its second segment now extends farther dorsad. The third segment 

 (13) is narrow. The ganglia retain their primitive intersegmental po- 

 sitions. 



Embryo of Seventeen Somites. — (Plate XXXVI.) The reduction of 

 the optic A'esicle (1) continues. The telencephalon (19) forms a promi- 



3 In this respect this embryo corresponds closely to the four-millimeter sheep embryo 

 figured by Neiimayer — Studien zur Entwickelungsgeschichte des Gehirns der Saiiger- 

 thlere. Festschrift ziim Siebenzlgsten Geburtstag von Tar] von Kuppfer. Taf. XLVIII, 

 fig. 4. .Tena. ISOfi. 



