[332] Antea, pp. 394-396.
[333] Perhaps the body I am describing is not identical with Leydig's posterior suprarenal body. I do not, as mentioned above, feel satisfied that it is so from Leydig's description.
[334] Quarterly Journal of Microscopic Science, October, 1874. [This edition No. V.]
CHAPTER XII.
The Organs of Excretion.
The earliest stages in the development of the excretory system have already been described in a previous chapter[335] of this memoir, and up to the present time no investigator, with the exception of Dr Alex. Schultz[336], has gone over the same ground. Dr Schultz' descriptions are somewhat brief, but differ from my own mainly in stating that the segmental duct arises from an involution instead of as a solid knob. This discrepancy is, I believe, due to Dr Schultz drawing his conclusions as to the development of the segmental duct from its appearance at a comparatively late stage. He appears to have been unacquainted with my earlier descriptions.
The adult anatomy and later stages in the development of the excretory organs form the subject of the present chapter, and stand in marked contrast to the earlier stages in that they have been dealt with in a magnificent monograph[337] by Professor Semper, whose investigations have converted this previously almost unknown field of vertebrate embryology into one of the most fully explored parts of the whole subject. Reference is frequently made to this monograph in the succeeding pages, but my references, numerous as they are, give no adequate idea of the completeness and thoroughness of Professor Semper's investigations. In Professor Semper's monograph are embodied the results of a considerable number of preliminary papers published by him in his Arbeiten and in the Centralblatt. The excretory organs of Elasmobranchii have also formed the subject of some investigations by Dr Meyer[338] and by myself[339]. Their older literature is fully given by Professor Semper. In addition to the above-cited works, there is one other paper by Dr Spengel[340] on the Urinogenital System of Amphibians, to which reference will frequently be made in the sequel, and which, though only indirectly connected with the subject of this chapter, deserves special mention both on account of the accuracy of the investigations of which it forms the record, and of the novel light which it throws on many of the problems of the constitution of the urinogenital system of Vertebrates.
Excretory organs and genital ducts in the adult.
The kidneys of Scyllium canicula are paired bodies in contact along the median line. They are situated on the dorsal wall of the abdominal cavity, and extend from close to the diaphragm to a point a short way behind the anus. Externally, each appears as a single gland, but by the arrangement of its ducts may be divided into two distinct parts, an anterior and a posterior. The former will be spoken of as the Wolffian body, and the latter as the kidney, from their respective homology with the glands so named in higher Vertebrates. The grounds for these determinations have already been fully dealt with both by Semper[341] and by myself.