326 A. T. MASTEKMAN. 



municates with a system of intercoelomic sinuses^ the primitive 

 vascular system. 



The four coelomic pouches all retain the muscular, repro- 

 ductive, and nutritive functions appertaining to the primitive 

 coelom. 



The axis of symmetry is perpendicular, and the central 

 nerve-ganglion is situated at the termination of this axis in the 

 direction of motion. 



I would specially emphasise the fact that the raesoblastic 

 pouches are evolved in this stage (Protaxonia, Hatschek), 

 so that their segmentation into four is due to radial deter- 

 mining factors, and it follows from this that they cannot strictly 

 be regarded as " paired." This segmentation of the mesoderm^, 

 for reasons given below, we may term the '^archimeric 

 segmentation." 



With horizontal motion forwards the axis of symmetry is 

 replaced by a perpendicular plane of symmetry in the direction 

 of motion. The Heteraxonia (Hatschek) stage is now com- 

 menced, and, as has been shown by Sedgwick, the blastopore 

 elonsrates and forms mouth and anus. 



Under these conditions the mesoblastic '' archimeres" are 

 placed under fresh symmetry-determining factors, and they 

 are now no longer under the same environmental conditions. 

 In woodcuts 9, 10, and 11 are seen three views of this stage 

 (Stage Ti). The new mouth and anus are almost completed, 

 and it is now seen that one of the archimeres (1) is pre-oral 

 in position, two are lateral (2, 2'), and the last (3) is now 

 post-anal. 



The first of these (the protomere) is now subjected to the 

 environment incidental to the front end of the body and to 

 the pre-oral position. No longer in a suitable position with 

 respect to the digestive part of the gut, it will lose its nutritive 

 and digestive function, which implies also a loss of that of 

 reproduction. In other words, its vegetative functions will be 

 lost whilst its animal functions will be markedly developed. 

 This protomere is evidently homologous with the pre-oral 

 lobe, hood, proboscis, &c., of so many of the Ccelomata. It 



