1896. NOTES AND COMMENTS. 291 



structures that occur not only in vertebrates but in invertebrates, and 

 even in Limulus itself. The gill-slits in the anterior region of the 

 alimentary canal are supposed by Dr. Gaskell to be the original spaces 

 between the concresced limbs ; why then, asked Mr. Weldon, are 

 they formed in a vertebrate as slits gradually appearing in a con- 

 tinuous wall, and why is there no trace of them in the post-branchial 

 region ? More fundamental was the objection so clearly stated by 

 Mr. MacBride that in the simplest type of vertebrate development 

 with which we are acquainted (that oiAmphioxus) the alimentary canal 

 is formed by invagination, precisely as it is in the arthropod Lucifev, 

 whereas the nerve-cord is formed much later by a simple layer of cells, 

 over which a sheath of non-nervous cells grows as a protection. 



Out of all these difficulties Dr. Gaskell extricates himself by 

 denying the fundamental postulates of embryology, and by setting 

 aside deep-seated homologies accepted by all comparative anatomists, 

 whatever their peculiar heresies may be, in order to set up for himself 

 a new method of morphological investigation, or rather, as he com- 

 placently informed us, a very antiquated one revived. The 

 coincidences of adult structure on which he lays such stress are 

 coincidences of a character familiar to those who study more than one 

 group of animals, and are often of very superficial nature. It is, for 

 instance, not to be wondered at that a transverse section of the thyroid 

 should resemble that of the genital duct of a scorpion, for as 

 Professor Weldon remarked, similar glandular arrangements are not so 

 rare among invertebrates. Nor can much stress be laid on the 

 presence of a chitinous layer in the skin of Ammoccetes, when we 

 observe that this is not outside the epidermis, as it is in Arthropoda. 

 As for the limbs of PtericUhys being morphologically comparable with 

 those of Limulus or the Merostomata, this was stoutly denied by our 

 chief authority on the subject, Dr. Traquair ; while Mr. Bather, 

 besides dwelHng on the difficulty of the transmutation of a highly 

 specialised type of structure into another highly specialised type with 

 totally different physiological relations, insisted that, if this were 

 really so, the absence of the well-developed connecting links, with 

 their presumed chitinous exo-skeletons, from the record of the rocks 

 was almost inconceivable in face of the fact that the broken ends of 

 the supposed chain were so fully preserved to us in the Silurian and 

 Devonian periods. 



Finally, Mr. Hoyle summarised what evidently was the view of 

 most zoologists present, in his parable of the medical student who, 

 chancing on a cuttle-fish, wrote a learned and elaborate paper clearly 

 demonstrating that every one of its structures was homologous with, 

 and the immediate precursor of, the organs of a typical vertebrate. 



Cell Facts and Cell Theories. 

 Yet another discussion, this time between zoologists and botanists 

 (why not also physiologists ?), was that on the Cell, initiated by 



