ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 427 



observes that the endodermal origin of the notochorcl, first described 

 by Balfour for the Selachii, has now been seen in Ampldoxus, 

 Petromyzon, the Teleostei, Urodela, Lacertilia, and Mammalia. So too 

 in the Tunicata the chorda is of endodermal origin ; and, on the 

 whole, Balfour's view is completely substantiated. The archenteric 

 cavity, which jorimitively arises by invagination, is partly converted 

 into the permanent digestive canal, and is partly constricted off. The 

 fore-gut is early differentiated into an anterior pharyngeal and 

 branchial portion, while the hinder part gives rise to the liver, heart, 

 and head-kidneys. In the former region the first differentiation is the 

 development of the branchial clefts, which arise as endodermal 

 diverticula, dividing the mesodermal plates of the head and passing to 

 the ectoderm. Eight pairs of clefts become developed ; the first pair 

 disappears altogether, and the embryonic arrangement found in the 

 Selachian Heptanchus is consequently highly primitive ; to the hypo- 

 thetical stem-form with eight clefts we may apply the term of Odotrema. 

 The arrangements seen in Petromyzon afford no support to the suppo- 

 sition that any vertebrate ancestor had gill-clefts in front of the 

 present first pair. So again, the mouth, which is a depression of the 

 ectoderm, would appear to be the primitive vertebrate mouth ; there 

 are no indications of any other, and there is no outpushing of the 

 endoderm, by which process a mouth must have been formed, if it has 

 not arisen from a depression of the ectoderm. 



The only glandular organs found in the embryonic lamprey are 

 the liver and the thyroid, and they are by no means well developed. 



As organs developed from the ectoderm we have (1) the integu- 

 ment, which during the whole of the embryonic life is unilaminate ; 

 its cells are high and somewhat irregular in form ; they are strikingly 

 similar to those found in the Triton at the same period, but are not 

 so regularly arranged. Further observations confirm the view that 

 the upper germ-layer primitively has its cells in a single layer. 

 (2) The central nervous system. Here the author finds himself in 

 agreement with Calberla, whose observations, however, were confined 

 to the earlier stages. Although the medullary tube exhibits a different 

 development in Petromyzon to that which is seen in other vertebrates, 

 the difference is only apparent. In the higher vertebrates (the 

 Teleostei excepted) the medullary tube is formed by the development 

 of two folds, which grow together and so enclose a canal. In the 

 lamprey the folds are appressed, and the medullary tube forms an 

 inwardly projecting knob, which, when it is separated off from the 

 ectoderm, is at first a solid tube, and only becomes hollow by the out- 

 growing of its cells. As in the Selachii (Balfour), the wall of the brain 

 is, at first, everywhere equally thick, and there are no special thickened 

 or thinner regions. The first sensory organ to appear is the auditory, 

 and it lies just in front of the first primitive vertebra, and is not, as 

 in the Selachii, separated from it by some distance ; but it is to be 

 noted that in the lamprey the whole brain is exceedingly small. One 

 of the most striking peculiarities in this organ is that for some time 

 there is no tendency to folding ; when it does occur it appears to be 

 due to the sudden increase in growth of the median portion. Owing 



