Charles Sedgwick Minot 89 



missure, s. c, there is also a considerable accumulation of differentiated 

 nervous tissue, indicated in the figure by shading with lines. This 

 material does not extend across from side to side but is interrupted by 

 a deep and rather narrow cleft in the morphological median plane — see 

 section 299 — as can be readily observed in transverse and frontal sec- 

 tions of this and earlier stages. The section figured passes through 

 the mouth of the epiphysis but does not pass through the cleft men- 

 tioned, hence the illustration gives a somewhat false impression. The 

 thickenings in question are continuous with the wall of the fore-brain; 

 they were termed tuhercnia intermedia by Gottsche, 35.1, walls of the 

 thalamencephalon by F. M. Balfour, and by recent writers are often re- 

 ferred to as the ganglia liahenulo'. I have not pursued the study of 

 these structures further. I will only remark in passing that the term 

 ganglion hahenuJce does not appear to correspond with any morphological 

 conception sufficiently clear to be valuable in comparative anatomy. 

 During the earlier stages the mesodern between the epidermis, Ec, and 

 the brain has been steadily growing and has now reached considerable 

 proportions, but the summit of the epiphysis, Ep., still lies relatively 

 near the outer skin and the organ consequently projects far above the 

 brain; it has begun to curve forward preparatory to its elongation ros- 

 trad. As regards the superior commissure, s. c, it is noteworthy that 

 it is now nearly as large as the posterior. As we ascend the vertebrate 

 series the posterior commissure increases in size and importance, but 

 the superior commissure is persistent occurring even in mammals. The 

 velum, V, has distinctly the character of a choroid plexus, being rich in 

 blood vessels, and bearing irregular villous outgrowths, the beginnings 

 of which can be seen in Fig. 8. The velar villi are much more devel- 

 oped laterally and their formation is spreading forward around the 

 sides of the paraphysal arch, a fact wliich I regard as of fundamental 

 importance for our final morphological interpretation. 



The last stage I have been able to investigate is an embryo Acanthias 

 of 86.0 mm. which is in a somewhat imperfect state of preservation, 

 the epidermis being in part lost, entirely so in the part drawn, although 

 indicated in the figure; the brain had shrunk a little; and as the epen- 

 dyma had withdrawn from the mesenchyma on both sides of the velum 

 that structure appears abnormally thick, but there seem to be no im- 

 portant distortions. The anlage of the skeleton, sic, is now clearly de- 

 fined in the mesenchyma on the same level as the upper vesicular end 

 of the epiphysis, Ep.; the stalk of the same is bent and has begun to 

 elongate forwards above the brain thus making a distinct approach 

 towards the adult condition described by Ehlers and Cattie. The para- 



