xii SCIENCE PROGRESS. 



true embryo, and comparing it with the Cyathozooid of Pyrosoma, he argues that the 

 ancestors of Salpa must have been developed frcmi a yolk-laden egg. He seeks to show 

 that the atrium of Salpa is homologous with that of the ordinary Ascidians, and that 

 its ancestors further possessed numerous stigmatic " gill-slits " of the Pyrosoma type. Arguing 

 along these lines, rejecting his earlier surmise that Salpa is the descendant of a pelagic free- 

 swimming organism, he points out that it owes all its distinctive structural peculiarities to a 

 sedentary life, and deduces the conclusion that Salpa is related to the Appendicularia indirectly 

 through a fixed ancestral form, and not directly as Herdman has supposed for both Salpa and 

 Doliolum. 



In this generalisation the author is supported by his pupil, Mr. M. M. Metcalf, Fellow 

 of the Johns Hopkins University, who contributes a subsidiary monograph upon " The Eyes 

 and Sub-Neural Gland of Salpa ". The salient feature of Mr. Metcalf's very careful piece of 

 work is the discovery that the ganglion of Salpa may be resolved into a dorsal portion 

 homologous with the adult Ascidian ganglion, and a ventral which represents the ventral wall 

 of the visceral portion of the larval Ascidian nervous system, which gives rise to the sub-neural 

 gland. Upon this basis he is led to deny the homology between the Salpa eye, and that of 

 the Tunicate larva and the pineal eye of the higher vertebrata, believing the first-named to 

 be the derivative of a portion of the nervous system not represented in the Ascidian tadpole 

 and the vertebrata. 



Dr. Brooks forcibly attacks the belief in the necessity of a physiological explanation of 

 metamerism, which has entered so largely into recent zoological conceptions and theories of 

 the inter-relationships of the higher groups of Metazoa. Restricting himself to a discussion 

 of the origin of those pharyngeal structures especially characteristic of the Tunicata, the 

 author proceeds to deal with the 'Anneliden Hypothesis" of Dohrn in denunciatory terms, 

 and he leads up to the conclusion that metamerism of the mesoderm and body-cavity may 

 have resulted from the duplication of a single pair of ccelomic pouches probably acquired by 

 the ancestors of the vertebrata after the divergence of the Tunicata. This definite attitude on 

 the part of a worker so competent to assume one is very welcome, now that the ancestry of 

 the vertebrata is much under discussion ; and both in his arguments and evident desire to 

 guard against the dangers of a too seductive " degeneration theory" the author has our full 

 sympathy. Advocating the above-mentioned views, he reverts to the conception of the 

 primitive nature of pelagic larva?, and opposes that of the secondary origin of pelagic 

 animals, by deviation and specialisation, from deeper living ancestors. Working along 

 these lines, he enters into a discussion of "The Origin of the Chordata, considered in its 

 relation to Pelagic Influences ". In the course of this and other portions of the work we meet 

 with an inclination towards the recapitulation theory of development, and that on what 

 appear to us sound lines. The conclusions are reached that the primitive vertebrates lived 

 upon or near the bottom, that the earliest steps in the evolution of the vertebrate classes were 

 made there, and that all the Metazoa have still more remotely descended from small pelagic 

 ancestors. Much of that portion of the book dealing with topics in which these conclusions 

 centre, might well have been condensed and omitted ; and it is irritating to find in a treatise 

 so voluminous, and so well written that it is in parts almost racy, whole passages needlessly 

 reiterated, and lengthy analogies to the affairs of daily life repeated word for word. 



Among the more striking details recorded is the discovery that the surface of the Salpa 

 body is delicately sculptured, and unequally thickened, apparently for purposes of support 

 and restoration to shape after contraction (facts which are shown to have an important bearing 

 on synonomy). The " follicle cells " are proved to be morphologically egg cells which do not 

 become fertilised, and the embryo is shown to be "blocked out" in them in detail. These 

 cells, however, do not become converted into the tissues of the definitive embryo, for they are 

 used up as food for the true embryo cells, some of them wandering into the blastomeres 

 during segmentation, others being said to migrate even into the ovarian eggs before 

 fertilisation, to give rise to the much discussed "test-cells" of earlier investigators. For 

 both these follicle cells and the placenta a nutritive function is claimed, and the author 

 discards, on sufficient grounds, the notion that the placenta is a respiratory organ, and in any 

 important respect comparable to that of the mammalia. Dr. Brook's work on the follicle cells 

 has been already confirmed by Heider, 1 and as Mr. Metcalf s contribution passed through the 

 press, Goppert published '- a paper on the eye of Salpa, which peculiarly agrees with his in 



1 Abhandlg. Senckcnbg. Naturf. Gesellsch., bd. xviii., p. 367. 

 - Morpholg. Jahrb., bd. xix. , p. 250. 



