330 ANNALS NEW YORK ACADEMY OF SCIENCES 



nent convexity in front of the optic vesicle. Near its upper limit there 

 is a point where the ectoderm is still adherent {S). The area which we 

 have termed infundibular region (IS) is ver}^ large and has a pointed 

 apex directed caudad. The mammillary region (77) has increased in 

 size but otherwise closely resembles that of the fourteen somite embryos. 

 The same is true of the mesencephalon (i-i). The hindbrain is markedly 

 enlarged in its vertical diameter. The pontine angle has increased in 

 prominence. The first hindbrain segment (31) is broad and prominent; 

 the second (12) is subdivided by a deep sulcus; its posterior moiety 

 {12b) has a considerable vertical extent but is now fusing dorsally with 

 the third segment (13) which remains narrow but has increased greatly 

 in height, forming indeed the apex of the pontine angle. The quintal 

 ganglion (J) retains its intersegmental position, but that of the pro- 

 fundus (3a) is beginning to shift caudad and is now in part attached to 

 the second segment (12a). The cranial extremity of the myelencephalon 

 has increased markedly in vertical diameter and is beginning to be 

 assimilated to the hindbrain; it shows a prominence of the roof at its 

 commencement which is separated by a depression from the remainder 

 of the roof-plate. This embryo showed some degree of side-to-side com- 

 pression with resulting diminution of the relief of its lateral walls and 

 an exaggeration of the projections in the dorsal and ventral midline. 



Embryo of Nineteen Somites. — (Plate XXXVII.) The forebrain is 

 defined by a well marked anterior isthmian constriction (22) which is 

 practically in line with the venter of the hindbrain. The tbalaman- 

 cephalon (16) has enlarged and is separated from the telencephalon (19) 

 by a shallow but definite furrow (20) extending from the dorsal mid- 

 line, obliquely over the lateral surface of the brain to the depressed area 

 immediately behind the optic vesicle where the thalamencephalon merges 

 into the relief of the regio mammillaris (17). The latter has not increased 

 in size and is separated from the regio infundibularis (IS) by a shallow 

 furrow; it is also distinguished by its greater lateral prominence. The 

 remainder of the forebrain comprises the optic vesicles and an ectoptic 

 zone surrounding them. The latter has greatly enlarged in its dorso- 

 cephalic portion, the telencephalon (19), which now forms the extremitv 

 of the brain. The regio infundibularis shows but a moderate increase 

 in size. Xew conditions are initiated ventral to the optic vesicles, which 

 now begin to retreat from the margin of the brain, leaving a narrow- 

 strip of tissue through which the infundibular region and telencephalon 

 are continuous. This condition is more clearly shown in the model of 

 the lumen. 



The midbrain (lo) is little changed as yet but is beginning to show 



