DEVELOPMENT OF HALISAEOA LOBULARIS. 609 



Oil their inner face^ unites with them to become the hypo- 

 blastic layer of the resulting bilamellar wall. In such a form 

 as that represented by the serial sections, figs. 27 and 32, the 

 former showing the blastopore, such a course of development 

 can be very simply understood; but there still remain other 

 cases in which more serious difficulties are encountered, for it 

 appears quite possible that more than one growing fold may 

 succeed in pushing its way into the interior, and so acquire a 

 hypoblastic relation. The instructive series of sections shown in 

 figs. 33 to 38 serve to illustrate such a case. Had one re- 

 mained content with a single section, such as either fig. 33 or 

 34, one would certainly have pronounced for a definite gastrula 

 stage, and the remaining sections would have been regarded as 

 exceptional or abnormal. A study of the entire series proves, 

 however, that in the case of several of the folds the adjacent 

 walls of each have grown together and given rise to a bi- 

 laminate wall, both layers of which are of the same nature, i.e. 

 hypoblastic. A similar union of the invaginating layer is shown 

 in fig. 39, taken from a diff"erent embryo. 



Notwithstanding all these diflferences in detail, and whether 

 epiblast unite with epiblast, or hypoblast with hypoblast, the 

 course of development invariably leads to one constant result, 

 i.e. the formation of a folded sac, the walls of which are in- 

 variably composed of two layers of similar cells, with nothing 

 except an almost imperceptible residuum of structureless 

 blastema interposed between them. Probably as development 

 proceeds beyond the stages we have observed, other cells 

 become detached from one or both of the layers of the wall, 

 and then wandering into the blastema, increased by growth, 

 give rise to the mesoderm. 



We have next to describe the history of the constituent 

 cells of embryo. In the commencing gastrula (fig. 18) these 

 are arranged in a single layer side by side to form the walls of 

 the sac. They are longer than broad (0004 mm. in diameter, 

 0"01 mm. long), with a more or less oval outline when viewed 

 sideways (fig. 20«), but polygonal (by appression) when seen 

 end on (fig. 20). A spherical nucleus (0*003 mm. in diameter) 



