556 BEPORT— 1882. 



tion ^ of the primitive character of this group was that increased interest should 

 attach to all researches on its embrj^ology. To an introductory account of the 

 embryology of Elasmobranchs - Balfour o^Yed, I believe, his fellowship at Trinity 

 College, and from that time onwards until 1878 he pursued the investigation at 

 Naples and in Cambridge. The collected results appeared in 1878, as ' A Mono- 

 graph on the Development of Elasmobranch Fishes.' No research upon a limited 

 group ever contained more numerous or more wide generalisations, extending over 

 the whole domain of vertebrate embryology. I may dwell for a few moments upon 

 some of its most interesting sections. 



The structures which we are now familiar with as ' head-cavities ' are described 

 for the first time, and named ; their relation to the cranial nerves and their resem- 

 blance or equivalence to the muscle plates of the body are pointed out ; and Balfoiu- 

 seizes upon their value in throwing light upon the great problems of the segmen- 

 tation of the head and the segmental value of the cranial nerves. In particular 

 the 5th nerve and the 7th, with the auditory, are specified as the segmental nerves 

 of the mandibular and hyoid segments. The short, but very important, notice of 

 the sympathetic system showed that its ganglia developed on branches of the 

 spinal nerves, and that it was therefore a product of the epiblast.^ The primitive 

 feature? of the mesoblast and notocord and their hypoblastic origin are de- 

 scribed,'' and furnish material for the comparison afterwards instituted in the 

 'Comparative Embryology' * between their development in Elasmobranchs and 

 their still more primitive origin in Amphioxus, as diverticula of the archenteron. 

 A very able chapter on excretory organs concludes this monograph. This subject 

 had engaged Balfour's attention very earlj', and his introductory account of Elas- 

 mobranch Development contains his discovery of segmental organs in Elasmobranchs, 

 — a discovery made independently but simultaneously by Professor Semper. These 

 organs are shown to develop in the mesoblast, and are compared with the seg- 

 mental organs of annelids. 



A paper published in 1876 gives a singularly clear and thorough resume of our 

 knowledge of the development of the urino-genital system ; and the diagrams there 

 given, illustrating the homologies of the male and female urino-genital organs, are 

 wonderfully simple and instructive. Shortly after the publication of this paper, 

 Balfour became a Fellow of the lloyal Society, from which he received a Royal 

 Medal in 1881. 



Among the interesting points that Balfour had made clear in connection with 

 the spinal nerves of Elasmobranchs, was the fact that the anterior and posterior 

 roots arise alternately, and not in the same vertical plane. He sought for an ex- 

 planation of this in Amphioxus at Naples, in 1876. Owsjannikow and Stieda 

 had discovered that the nerves of the opposite sides in Amphioxus arise alternately, 

 and Stieda further stated that the nerves of the same side arise alternately from the 

 dorsal and ventral corners of the cord. Stieda considered that two adjacent nerves 

 were together equivalent to a single spinal nerve of higher vertebrates. Balfour * 

 found no trace of difierence of level in the origin of nerves on the same side, i.e. he 

 denied the existence of ventral or anterior roots ; and afterwards, in investigating 

 the cranial nerves of higher vertebrates, and being unable to find any trace of an- 

 terior roots, he framed the bold hypothesis ' that the head and trunk had been 

 differentiated from each other at a time when mixed motor and sensory posterior 

 roots were the only roots present, and that cranial and spinal nerves had been inde- 

 pendently evolved from a common ground-plan. 



Balfour's investigation of the development of the ovary was incomplete when 

 his work on Elasmobranchs appeared ; and he continued to work at this subject, 



' Gegenbauer, Das Kopfsltelett der SelacMer, 1872. 



" Quarterly Jcnirnal of Microscopical Science, vol. xiv. 1874. 



' Elasmobranch Fishes, p. 172. 



* Ibid. pp. 49, 85, 92, 104. 



' Comparative Embryology, vol. ii. pp. 243-246. 



* Journal of Anatomy and Physiology, vol. x. 1876. 



' Elasrtwhranch Fishes, p. 193, Comparative Embryology, vol. ii. p. 380. 



