390 ARTHUli T. MASTERMAN. 



takes place as rapidly as the formation of the groove itself, 

 so that there caunot (in this species) be found a part in 

 which these two layers are fused; at least, I have failed to 

 find a section exhibiting this condition. The ventral 

 groove would thus appear to be a structure merely indicating 

 the line of fusion of the blastoporic lips, and not correlated 

 with an internal line of junction between hypoblast and 

 epiblast. Fig. 27 shows that internal separation of the two 

 layers proceeds forwards with the fusion of the blastoporic 

 lips. 



Formation of the Mesoblast. 



A determination of the true formation of the mesoblast in 

 Phoronis was the primary object of this investigation. 

 Unfortunately, owing to the fact that the material was not 

 preserved with a view to embryological work, I am not in a 

 position to demonstrate every detail of the mesoblast forma- 

 tion in P. Buskii to the extent which I could wish, and I 

 was teinpted to withhold these results till the opportunity of 

 working out another species with properly prepared material 

 presented itself ; but as the lesser details may as likely as not 

 differ in the two species, I propose to give a brief account of 

 the results to which I have been led by the examination of 

 sections of P. Buskii. 



We have already seen that up to the stage depicted in 

 fig. 12 and in fig. 26 in section, there is no trace of mesoblast 

 in any part of the embryo. 



In fig. 27 the archenteron is seen to be extending* into the 

 anterior part of the embryo, which is at this stage rapidly 

 growing. 



The hypoblast level with this forward extension of the 

 ai-chenterou bounding it laterally and anteriorly is thinner 

 than the rest (fig. 27, p. c), and in places has more nuclei. 

 At a slightly later stage, in coronal section, this anterior part 

 of the archenteron is seen to have still thinner walls, and to 

 grow backwards as a pair of lateral lioi us. Its cavity is still 



