EAELY STAGES IN DEVELOPMENT OF BALANOGLOS3U3. 223 



It may be useful to sura up the principal points of internal 

 structure which are to be observed in Stage E. Of these the 

 most important features are : — (1) The commencement of the 

 nervous system as a median dorsal thickening of the epiblast 

 in the region of the collar; and (2) the formation of the meso- 

 blast. Briefly to recapitulate the latter process, it consists of 

 (a) the constricting ofFof the anterior portion of the archenteric 

 cavity from the remainder to form a single median impaired 

 mesoblastic pouch, which secondarily sends back a pair of hollow 

 outgrowths placed one on either side of the gut ; (,6) the for- 

 mation of a pair of cavities in the archenteric wall in the region 

 of the collar whose lumina open into the lumen of the gut ; 

 (y) the appearance of a pair of archenteric diverticula in the 

 posterior region of the trunk. 



It will therefore be observed that the process of formation of 

 the first body cavity is constituted by a direct specialisation of 

 the hypoblastic wall^ and that while the posterior pair of meso- 

 blastic somites are actual archenteric diverticula^ the origin of 

 the anterior pair partakes of the characters both of archenteric 

 diverticula and of delamination from the hypoblast. These two 

 modes of origin of the mesoblastic somites are, of course, essen- 

 tially the samCj differing from each other merely in degree. 

 This case is perhaps interesting as affording an illustration of 

 these two methods both occurring in the same animal, and the 

 combination of the two processes in the case of the middle 

 somites is especially noteworthy as representing the last stage 

 of the phylogenetic transition from hollow archenteric out- 

 growths to mere plates delaminated from the hypoblast. 



Fig. 35 represents a longitudinal vertical section through a 

 larva slightly before Stage E is reached, in which the nervous 

 system is not yet formed even in a rudimentary condition. 

 As the plane of the section passes through the middle line the 

 middle and posterior portions of the mesoblast are not shown. 

 The relations of these to the regions of the body are illustrated 

 by fig. 40, which is a diagram of a horizontal median section of 

 such a larva. 



The Separation of the ]Mesoblast. — The next changes of 



