1902.] CONKLIN — EMBRYOLOGY OF A BRACHIOPOD. 47 



not quite impossible, to work out the cell lineage of Terebratulina, 

 even with an abundance of most favorably preserved material. 

 With the material at my disposal such work was wholly out of the 

 question. In the blastula, even at the time when gastrulation 

 begins, one is struck by the great uniformity in size and quality of 

 all the cells. I have found it quite impossible to distinguish any 

 difference between the cells which invaginate and those which do 

 not until after the gastrulation is well advanced. 



III. Gastrulation and Formation of Body Layers 

 AND Cavities. 



Gastrulation occurs by typical invagination, and at the time 

 when the infolding begins there is no difference in the cells at the 

 two poles (Figs. 13 and 37). The infolding continues until the 

 inner layer comes into contact with the outer one and the blasto- 

 coel is entirely occluded (Fig. 14 et seq.). During this process 

 there is a decided change in the character of the cells of the inner 

 layer ; they become very much shorter and henceforth are cuboid 

 or rounded in shape ; the cells of the outer layer remain columnar 

 in shape and are very long, so that the ectoderm is quite thick. 

 Large deeply staining granules are found at the free ends of all the 

 cells, both those which are invaginated and those which are not, 

 and in the invaginated cells these granules are so dense that they 

 frequently obscure the nuclei and cell boundaries. In the ectoderm 

 these granules lie on the outer side of the nuclei (Fig. 37 et seq. 

 Fg), while the inner ends of the cells are left free from gran- 

 ules and nuclei and hence are very clear. I suggest that these 

 granules may be associated with the cilia, which in life cover the 

 embryo and line the enteron and coelom (see Morse, '73). 



Almost as soon as the inner layer comes into contact with <-he 

 outer one — /. <?., when the infolding is complete — the innermost por- 

 tion of the archenteron becomes slightly constricted from that 

 portion lying nearer the blastopore. This constriction is deepest 

 anteriorly, least marked posteriorly, while it is about equal on the 

 left and right sides of the archenteron (Figs. 14, 15, 16, 38). On 

 the anterior side this constriction develops into a partition wall, 

 which grows downward and backward into the archenteron, shutting 

 off the enteron above from the enteroccel below (Figs. 16, 20, 

 42fz and 42^). So far as I am able to determine this partition wall 



