334 (^^^' ' [Vol- I- 



optic stalk. The relation of these parts and the development of 

 the lens is illustrated in the figures just referred to. 



The first development of the ear (^) and the olfactory 

 organs {No) is shown in the figures 59, B, PI. XVI., and 41, 

 C, PI. XV. Both organs appear first as thickenings of the 

 epiblast, which gradually become hollow, rounded depressions. 

 The ear becomes later constricted off from the epiblast as a 

 hollow spherical body, with its median wall touching the hind- 

 brain (Fig. 24, C). At the same time its nerve reaches it from 

 before. The nasal thickening receives its nerve before there 

 is any appearance of a depression (41, C), The depression, 

 which appears later, always remains externally open. 



By the time the above-described changes have taken place, 

 the brain has reached the stage of development which is illus- 

 trated in series C (Figs. 2, C, and 24, C, to 42, C). The line P-s, 

 Fig. 2, C, indicates the plane of the sections. This series shows 

 the rudiments of the anterior ten nerves of the head, with the 

 exception of the fourth and sixth, which have not yet appeared. 

 There is still no trace of nervous fibres in any part of the 

 brain or spinal cord. The hind-brain (Figs. 24, C, to 29, C) is a 

 slightly curved tube, with a triangular lumen much widened 

 anteriorly, while posterially it blends into the spinal cord. The 

 sides are thick, meeting almost at an angle ventrally ; dorsally 

 they become thinner, and from their edges a thin membraneous 

 roof is spread almost flat across the top. Section 30, C, shows 

 the narrow part between the hind-brain and mid-brain. From 

 this point forward the mid-brain extends as a rounded swelling, 

 and is cut almost transversely. The third nerve-pair springs 

 from it ventrally. In Fig. 35, C, appears the infundibular region 

 of the fore-brain (^FB.). A slight elongated constriction sepa- 

 rates the mid-brain from the fore-brain ; this is seen in Figs. 

 38, C, and 39, C. This constricted region becomes the thalamen- 

 cephalon, and in Fig. 42, C, is seen the dorsal constriction {c) 

 marking off the region of the thalamencephalon from the dorsal 

 swelling which later divides into the two lobes of the secondary 

 fore-brain (FB'). Sections of more advanced embryos, show- 

 ing the later growth of the brain, are figured at 6, PI. XII., 52, E, 

 PI. XV., and 62, F, and 63, F, Pi. XVI. The explanations of 

 the plates will show their relations to the parts just described. 



Previous to the stage represented in series C, there have 



