GENERAL INTRODUCTION. xxix 



e. g. the pole-cells of Chaetopoda ; (4) from the primitive streak behind the 

 blastopore in Peripatus, the same streak in Insecta or Spiders, which may 

 be partial or complete homologues of the streak in Peripatus l ; (5) from 

 the walls of diverticula of the archenteron or enterocoelic pouches as in 

 Balanoglossus, Sagitta and Brachiopoda. When it has a double source 

 it may be (i), supra, combined with enterocoelic pouches as in Echinoder- 

 mata, or with pole-cells as in Thalassema ; (3) supra, combined with entero- 

 coelic pouches as in Amphioxus ; (2) or (4), supra, combined with cells 

 derived from the hypoblast as in most Vertebrata. Whatever significance 

 these facts may have, the mesoblast gives rise to the muscular, connective 

 and skeletal, blood and lymph tissues, and very generally to the genital and 

 excretory cells. In Vertebrata, Cephalochorda, Arthropoda, some Vermes, 

 e. g. Chaetopoda, it is broken up into a series of paired segments, sometimes, 

 as in many Crustacea, obscurely marked, giving rise in the adult to a 

 metamerism or serial segmentation, the primitive origin of which is a 

 matter of doubt. In connection with the mesoblast or its segments is the 

 cavity, or series of cavities, known as body cavities or coelome, which are 

 not homologous throughout the Coelomata. In some Vermes a coelome 

 is absent, or represented by irregular spaces or gaps in the mesoblastic 

 tissues, as in Turbellaria, Trematoda, and Cestoda, but in other Coelomata 

 it is probable that it falls under one of the following heads, (i) It is an 

 archicoele, or remnant of the blastocoele as in the vascular system of 

 Nemertea, the head cavities of some segmented Vermes, e. g. Polygordius, 

 some Chaetopoda, the body cavity of Rotifera, and Dinophilus (?). (2) It is 

 a system of channels and spaces excavated in the mesoblast secondarily, 

 e. g. the principal portions of the coelome, or the vascular spaces in 

 Peripatus, and perhaps in Mollusca, or the whole of it in Arthropoda in 

 general. To this type the name metacoele might be applied 2 . (3) It is 

 an enterocoele, i. e. the persistent cavity of diverticula of the archenteron 

 (supra], as in Amphioxtis, Balanoglossus, Sagitta, Brachiopoda, and Echino- 

 dermata. There remain the coelomic cavities of Vertebrata, the isolated 

 nephridial pouches and genital ducts of Peripatus, the series of cavities in 

 segmented Vermes, the pericardium, nephridial, and perhaps genital cavities 

 of Mollusca. In these instances they are now usually regarded as entero- 



1 See on the primitive streak, Sedgwick, Q. J. M. xxiv. p. 79. and xxvii. p. 530; Haddon, 

 Introduction to the Study of Embryology, 1887, P- 4 1 > an ^ t^ 6 various references given in the Index 

 vol. ii. of Balfour's Comparative Embryology. 



2 Sedgwick uses the term pseudocode, but it would apply equally well to an archicoele, or to 

 a schizocoele, if there is such a thing, supposing his definition of a coelome to be accepted (Q. J. M. 

 xxvii. p. 533). 



