890 



SCIENCE. 



[N. S. Vol. XIV. No. 362. 



tion, R. S. Bergh, in 1885, maintained that the 

 segmented coelome of annelids is homologous 

 with the cavities of the gonad-follicles of pla- 

 todes and nemertines, and that the primitive 

 function of the peritoneal epithelium of anne- 

 lids was that of a germinal epithelium. The 

 same conclusion was independently reached by- 

 Meyer and developed by him in 1890, as a se- 

 quel to his embryological studies on Psygmo- 

 branchus, into a general theory, of which the 

 essential points are that the mesoblast-bands 

 (developed from the pole-cells) of annelids are 

 homologous, as a whole, with the paired gonads 

 of the platodes ; that by a change of function 

 many of the primitive germ-cells gave rise to 

 somatic mesoblastic elements ; that by this 

 process a ' secondary mesoblast ' arose ; and that 

 lastly, by a partial process of substitution, the 

 secondary mesoblast, in a greater or less degree, 

 took the place of the primary mesoblast of the 

 platode, which, however, still appears in the 

 ontogeny as the * larval mesenchyme ' and 

 some other structures. These last assumptions 

 were not mere guesses, but were based on care- 

 ful observations which showed that in at least 

 one larval annelid {Psygmobranchus) there are 

 two entirely distinct sources of mesoblast, 

 namely, a ' primary mesenchyme ' derived from 

 the ectoblast, and a 'secondary mesoblast ' de- 

 rived from the pole-cells, and forming the ' me- 

 soblast bands ' in the ordinary sense. The 

 former gives rise to the larval muscles, some 

 of which are only ten^porary or provisional 

 structures (including the protrochal ring-mus- 

 cle), and to some of the adult muscles, includ- 

 ing those of the gut, of the dissepiments, and 

 the circular muscles of the body-wall. The lat- 

 ter gives rise to the peritoneal epithelium, the 

 gonads, and the longitudinal muscles of the 

 adult. These results were supported by the 

 independent observations of Bergh and Vejdov- 

 sky, showing that in leeches and earthworms 

 also the circular muscles are of wholly differ- 

 ent origin from the longitudinal ones, the latter 

 alone arising from the mesoblast pole-cells. 



In the present paper Meyer reenforces his 

 position by a great number of new observations 

 on many different annelids, of which the most 

 thoroughly studied were Polygordius and Lopa- 

 dorhynchus. Although both these forms had 



already become classical objects through the 

 earlier work of Hatschek, Fraipont, Kleinen- 

 berg and others, Meyer brings forward an 

 almost bewildering profusion of new results for 

 both, which sustain and extend his earlier 

 conclusions on Psygmobranchus. In both forms 

 primary and secondary mesoblasts are wholly 

 distinct in origin and in fate ; in both, the meso- 

 blast bands (secondary mesoblast) give rise only 

 to the peritoneal epithelium, the gonads and 

 the definitive longitudinal muscles of the body- 

 wall ; in both there are several regions in 

 which mesoblast (primary mesenchyme) is inde- 

 pendently derived from the ectoblast. From 

 the primary mesenchyme are derived not only 

 the provisional larval musculature, but also an 

 important part of the definitive musculature, 

 namely, that of the gut, of the parapodia and 

 cephalic appendages, of the dissepiments, and 

 the circular and diagonal muscles (when pres- 

 ent) of the body-wall. Meyer thus shows that 

 although a part of the larval musculature (for 

 instance, the prototrochal ring-muscle) is un- 

 doubtedly of a provisional character, yet a 

 much larger part of it is retained in the adult 

 than has hitherto been supposed. This part is 

 assumed to have been derived from the platode 

 parenchyma, which has, as it were, been car- 

 ried over into the annelid organization. Among 

 many interesting special points may be men- 

 tioned the discovery of Si paratrochal r'lng-Taerwe ; 

 and the demonstration, both in Polygordius and 

 in Lopadorhynchus, of numerous true neuro- 

 muscular foundations — i. e., areas in which 

 nerve-cells and primary muscle-cells are pro- 

 liferated from common ectoblast areas, but it is 

 also shown with perfect clearness that Klein- 

 enberg was in error in maintaining the origin 

 of the secondary mesoblast-bands and the ven- 

 tral nerve-cords from such a common neuro- 

 muscular foundation. Interesting detailed 

 studies are given of the larval nervous system ; 

 Kleinenberg is shown to have been in error' 

 in deriving the germ-cells directly from the 

 ectoblast ; and the existence of Hatschek's 

 famous nephridial ' Langscanale ' is again 

 denied. 



Fortified by these new facts, Meyer reasserts 

 and further extends his original hypothesis, 

 giving a thorough and critical review of the 



