1902.] CONKLIN — EMBRYOLOGY OF A BRACHIOPOD. 51 



IV. Orientation of the Embryo and Establishment 

 OF Definitive Axes. 



It is well known that previous investigators of the embryology of 

 the brachiopods have found the proper orientation of the embryo 

 one of their most perplexing problems. The complete closure of 

 the blastopore at a relatively early stage and before many land- 

 marks have appeared by which its position relative to definitive 

 organs could be determined, as well as the fact that both the oral 

 and aboral sides of the embryo are very similar, has been chiefly 

 responsible for this uncertainty of orientation. However if one 

 studies carefully whole embryos of the critical stage when the last 

 remnant of the blastopore is disappearing, the proper orientation of 

 the embryo of a brachiopod is no more difficult than is that of any 

 other animal. 



In the stage shown in Figs. 23 and 24 the antero-posterior axis 

 of the embryo is well defined, while the remnants of the blastopore 

 are still present. The enlarged end of the embryo (to the right 

 in Fig. 24) is anterior and gives rise to the head, while the 

 posterior end (to the left in the figure) is narrowed and gives rise 

 to the peduncle. The point where the blastopore closed (Fig. 24, 

 Br) lies near the middle of the ventral side, while the blastopore 

 groove runs backward almost to the posterior end of the embryo. 

 Directly opposite the blastopore is a groove which runs transversely 

 across the dorsal side of the embryo ; this is the dorsal mantle 

 groove, and the prominent ridge anterior to it is the dorsal mantle 

 fold (Fig. 24, Md). If now Fig. 24 be compared with Figs. 20 

 and 16 it will be seen that the blastopore occupies the ventral-pos- 

 terior region of the embryo, and that the anterior pole of the 

 embryo is rounded while the posterior pole is pointed, the embryo 

 being flattened on its postero-dorsal side. The prominent ridge 

 opposite the blastopore in Figs. 16 and 20 corresponds with the 

 dorsal mantle fold in Fig. 24. A comparison of these three 

 figures further shows that the axis connecting the middle of the 

 blastopore v/ith the apex of the gastrula invagination (<?. g., E \v\ 

 Fig. 14) is ultimately bent on itself through an angle of more than 

 90°. It is difficult to say whether this bending of the gastrula 

 axis is chiefly due to the forward shifting of the blastopore on the 

 ventral side or to the forward shifting of the apex of the gastrula 

 invagination, since there are no points in the embryo which may be 



