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ORR. ' [Vol. I. 



long axis of the hind-brain. In the fully developed individual 

 this part of the mouth-roof lies nearly in the horizontal plane of 

 the long axis of the body, and the primary cranial flexure has 

 meanwhile increased. It is necessary, therefore, as the anterior 

 part of the head approaches the straight line of the main body- 

 axis, that the secondary cranial flexure should form a curve or 

 angle of about ninety degrees. The manner of this bending is 

 illustrated in Figs. 3 and 4, PI. XII. In the region of the bend, 

 the roof of the hind-brain is so thin that it offers comparatively 

 no resistance to the mechanical changes of position of the lateral 

 walls. Anteriorly and posteriorly, however, the walls are held 

 in position by the thickened roof. Fig. 4 shows how the hind- 

 brain {HB) bends ventral-wards, the ear being just behind the 

 apex of the bend. Fig. 3 shows how, at the same time, the dor- 

 sal edges of the lateral walls are distended laterally at the apex, 

 so that the roof of the ventricle presents the appearance of a 

 rhomboid. If we conceive a pliable body of the same shape as 

 the hind-brain, it is evident that by undergoing a similar bending 

 it would present the same appearance. In the hind-brain the 

 bend continues to increase. The posterior and anterior walls of 

 the rhomboid approach each other, the median angles dividing 

 the lateral walls becoming more obtuse, while the lateral angles 

 become more acute. Finally, the two anterior and the two pos- 

 terior walls lie approximately in two transverse straight lines and 

 nearly touch each other. The reduced lumen presents the form 

 of a cross. By this process the region of the elongated neuro- 

 mere between the mid-brain and fifth nerve-pair, becomes the 

 region of the cerebellum. The fifth nerve-pair springs from the 

 lateral angles produced by this bend of the hind-brain. The 

 ear, which now occupies a somewhat greater space, touches the 

 mid-brain. The hind-brain has naturally continued its growth' 

 during this period, but its principal changes of form are pro- 

 duced according to the mechanical laws of flexion. 



In some amphibian and teleost ^ embryos which I have ex- 

 amined, in various stages of early development, the process of 

 the secondary cranial flexure seems to be very much abbreviated 

 or almost obliterated. The first appearance of it consists in 

 lateral angular outgrowths, or distensions of the walls in the 



' I am indebted to Mr. C. Earle for kindly allowing me to examine his prepara- 

 tions of the different stages of the embryos of Ctenolabrus. 



