416 PROFESSOR LANRESTER. 



to the primitive cell-layers iu nearly all the Enterozoa, 

 leading eventually to the production of an apparently special 

 layer of cells between deron and enteron^ and to the forma- 

 tion of a cavity^ also placed between those layers^ and known 

 by Haeckel's term '' Coelom." 



I shall pursue the method already adopted of giving the 

 results of observation and reasoning, first of all, in the form 

 of a statement of the hypothetical course of dififerentiation 

 of the ancestral series, taking up the story of the Diblastula 

 with mouth and stomodseum just developed, but destined to 

 acquire anus and proctodaeum somewhat later. 



1 . Differentiation of a layer of fibres from the deep surface 

 of the deron. — The ectodermic cells, fulfilling, as Kleinenberg 

 has pointed out, the function of protective, tactile, and con- 

 tractile organs, now proceeded to differentiate each for itself 

 a contractile tail or appendix, as we actually observe in 

 Hydra. 



2. Delamination of these fibres as fusiform, contractile, and 

 skeletal cells. Just as at an earlier stage the digestive 

 portion of each primitive cell, separated by delamination 

 from the receptive tegumentary portion, giving rise to ecto- 

 derm and endoderm, so now the contractile fibrous appendices 

 of the ectoderm acquired each the characters of a separate 

 cell, and separating by delamination from the ectodermal 

 cells, formed a distinct hypoderic musculo-skeletal layer, in 

 fact, a primitive mesoderm or mesoblast. In this stage, how- 

 ever, the cells do not present that early independence which 

 is what defines the mesoblast. The musculo-skeletal cells of 

 the Zoophytes are deep layers of the ectoderm, and do not 

 take origin as a distinct " mesoblast " at an early period of 

 development. They are not, in fact, sketched out, their 

 progenitors are not marked off as a separate layer, in that 

 phase of development when all the cells of the embryo are 

 undifferentiated in appearance, and when the enteron is 

 beginning to be formed by invagination. This, however, is 

 the period at which the musculo-skeletal cells arise in higher 

 forms than the Zoophytes, and hence the name Triploblastica 

 applied to those groups. 



3. Precocious segregation of mesoderm and ente7^ic origin of 

 the calom. — This early independence of the middle cells of 

 the organism is traceable to two distinct causes or antece- 

 dents, which it is one of the main objects of the present 

 essay to set forth. These two causes are, firstly, the deve- 

 lopment of diverticula, or so-called '^ gastro-vascular^^ out- 

 growths of the archenteron, which eventually become pinched 

 off from the enteron, and form a distinct closed cavity, the 



