GENERAL INTRODUCTION. xxvil 



nuclei. Indeed in Peripatus cells are never distinctly delimited, the result 

 being a syncytium 1 . The Gastrula is derived either by invagination or 

 by differentiation of the yolk-cells. 



In some Coelenterata the Gastrula stage is attained either by im- 

 migration of cells from one or various points of the blastosphere into 

 the blastocoele, with subsequent differentiation of the immigrant cells, by 

 delamination of the inner ends of the blastospheral cells, or by a mixture 

 of the two processes. See p. 746, note i, pp. 752, 764, 800. The varia- 

 tions observable in closely allied genera, make it probable that the phe- 

 nomena as observed in these cases are of secondary origin, due perhaps 

 to a precocious formation of the endoderm. The name ' parenchymula ' 

 or ' parenchymella ' has been applied to the form where the central cavity 

 is filled with cells. 



There can be no doubt that segmentation and gastrulation, processes 

 which take place in every life-history or ontogeny, represent ancestral 

 stages in the evolution or phylogeny of multicellular animals. But at 

 the present time there is no such thing known as a sphere or blastosphere 

 leading an independent life and reproducing its kind, its component cells 

 united by a bond indissoluble without entailing death on each cell. The 

 same statement is true of the Morula and of the Gastrula. The claims 

 of the Mesozoa to represent a Gastrula are excessively doubtful ; those 

 of Haeckel's Gastreadae^ at present inadmissible. See pp. 817-8. 



Putting, the Mesozoa aside, the vast majority of multicellular animals 

 may be classified as Metazoa. The growth of the individual is compli- 

 cated by the formation of tissues and systems of organs. . Sensory and 

 nervous tissue, contractile tissue, supporting or connective tissue, localised 

 reproductive tissue, are differentiated in connection either with the epi- 

 blast s. ectoderm, and hyboblast s. endoderm of the Gastrula, or in part 

 independently of them. Two main divisions of Metazoa are recognisable, 

 the Coelenterata and Coelomata. 



The typical characters of the Coelenterata are as follows. The funda- 

 mental symmetry of the Gastrula is as a rule retained ; the vertical axis 

 passing through the blastopore persists ; if an anterior and posterior 

 extremity are distinguishable they are equal ; and the same is true of a 

 right and left side. The blastopore may close, or not be developed ; 

 the archenteron, except in Porifera, opens by a single principal aperture, 

 which is either a perforation of the two embryonic layers, or a distinct 

 involution of the epiblast known as stomodaeum, which may assume a 



1 This may be a very primitive condition ; cf. Sedgwick, Q. J. M. xxvii. 1886, pp. 515-30. 



