December 6, 1901.] 



SCIENCE, 



891 



literature and putting forward many ingenious 

 suggestions regarding the possible phylogeny 

 of the coelome, blood-vessels and musculature, 

 the origin of metamerism, and other deep-lying 

 morphological problems. Phylogenetic specu- 

 lations on embryological data are getting out 

 of fashion, and some of Meyer's conclusions 

 will doubtless meet with little sympathy on the 

 part of those whose interest in the historical 

 problems of morphology has suffered a tempo- 

 rary attack of paralysis through devotion to 

 more ' modern ' questions. Even the sceptical 

 reader, however, who will take the trouble 

 to examine Meyer's work with care, will not 

 be able to deny that the theoretical views are 

 everywhere held closely in touch with admir- 

 ably thorough and extended observation, and 

 constitute no mere inflated speculative system, 

 but a natural working hypothesis growing 

 directly out of the facts. 



In the present paper Meyer considers only 

 the larval development ; and his results form a 

 most important supplement to that of students 

 of cell-lineage, who have not, in general, 

 carried their work to a sufficiently late period to 

 determine the real relation of the germ-layers 

 to the adult body. It may be pointed out, 

 however, that the compai-ative study of cell- 

 lineage in platodes, annelids and mollusks has 

 steadily added weight to Meyer's original con- 

 tention of a double origin of the ' mesoblast,' 

 for it has shown that in the two higher groups a 

 ^ larval mesenchyme ' is often formed from cells 

 of the ectoblastic quartets, which are quite dis- 

 tinct from the pole-cells of the secondary meso- 

 blast, the latter (with the apparent exception 

 of Capitella) being always derived from a cell 

 of the fourth quartet (otherwise entoblastic). 

 The cell ancestry of the larval mesenchyme 

 thus agrees in a general way, though with 

 interesting modifications of detail, with that of 

 the mesenchyme (mesoblast) of polyclades, 

 which inevitably and independently suggests 

 the same view as that of Meyer, though from 

 a quite different point of view.* Meyer's ob- 

 servations render it in the highest degree prob- 

 able that, as the writer has suggested, mesen- 



*Cf. Wilson. ' Considerations on Cell-lineage and 

 Ancestral Reminiscence,' in Annals N. Y. Academy 

 of Sciences, XI., 1, 1898. 



chyme may arise from any of the three ecto- 

 blastic quartets ; for (not to mention the so- 

 called 'head-kidneys' of Nereis), such origin has 

 already been observed in the second and third, 

 and if the cell-lineage of Pulygordius and Lopa- 

 dorhynchus is of the same type as in other anne- 

 lids, as can hardly be doubted, the umbrellar 

 neuro-muscular foundations in these forms must 

 be derivatives of the first quartet. 



The gonad theory of the coelome, which 

 Meyer has done so much to advance, has made 

 a deep impression on morphology, as may be 

 seen, for instance, by reference to the admirable 

 review of the theories of the coelome by Ray 

 Lankester in the second volume of his ' Treatise 

 on Zoology,' which appeared last year ; and it 

 has made serious inroads on the widely ac- 

 cepted enterocoele theory. Whether the two 

 views can be reconciled is not to be determined 

 without further research ; for some of the most 

 important observations on which Rabl, Lan- 

 kester and others have relied in attempting to 

 trace the transition from the pole-cell ty{)e to the 

 enterocoele type {e. g., pole-cells in Amphioxus, 

 gut-pouches in Paludina) have been shown to be 

 erroneous. Meyer believes the enterocoele 

 type to be secondary ; Lankester accepts the 

 reverse view. Others have suggested the pos- 

 sibility that the two types have been distinct 

 from the beginning, and this has for years been 

 held open in the writer's advanced lectures on 

 zoology and embryology as a possible basis for 

 a division of the ' triblastic ' animals into two 

 parallel but independent series that diverged 

 further down than the platodes — a division 

 which, though entirely provisional, and as yet 

 without adequate basis, nevertheless brings 

 into order a surprisingly large number of facts 

 otherwise. difficult to reconcile. This is a ques- 

 tion for the future, and may be left with Lan- 

 kester's significant remark, that" When the cell- 

 lineage of mesenchyme and its tissue-products 

 has been cleared up we may be able finally to 

 put aside the hasty criticisms and phantastic 

 assertions of those who have grown impatient 

 over the slow and difficult task of cellular 

 embryology." Edmund B. Wilson. 



Publications of the University of Pennsylvania, 

 Astronomical Series. Volume I., Part III. 

 The work before us is Part III., of the publi- 



