1902.] CONKLIN — EMBRYOLOGY OF A BRACHIOPOD. 49 



is due in large part to the change in shape of the embryo, which 

 becomes flattened in a dorso-ventral direction and enlarged in its 

 other axes. Associated with the growth of the enteroccel is an 

 important change in the character of its bounding cells;- at first 

 they form a quite regular, cuboid or columnar epithelium (Figs. 

 14-16 and 40-42^), but as the enterocoel increases in size the 

 epithelium becomes less regular, particularly at the anterior end, 

 and here many mesenchyme cells come to lie in the cavity of the 

 enteroccel (Figs. 20, 21, 42^^, 43). Later such mesenchyme cells 

 are found generally throughout most of the coelom. The cells 

 bounding the enteron remain cuboid or columnar throughout the 

 development. 



With the flattening of the embryo and the closure of the blasto- 

 pore, the ventral wall of the enteron is brought into contact with 

 the ectoderm at the place where the blastopore closes (Figs. 24 

 and 44-47), and consequently the enterocoel is here divided into 

 right and left cavities, which however still communicate with each 

 other at the anterior end and open into the enteron posteriorly 

 (consult Figs. 43-47 which are transverse sections of an embryo of 

 the stage shown in Fig. 24, Fig. 43 being the most anterior section 

 drawn and Fig. 47 the most posterior). Very soon after this stage 

 the communication between the enteron and the enterocoel is com- 

 pletely cut off and the definitive coelom is thus formed, consisting 

 of two sacs, still opening into each other anteriorly and posteriorly 

 but separated throughout the middle region of the embryo (consult 

 Figs. 48-52 which are cross sections, in order from the anterior to 

 the posterior region, of an embryo of about the stage shown in 

 Fig. 28). 



The gastrulation and formation of body cavities in brachiopods 

 has been observed heretofore only by Kowalevsky. A comparison 

 of the method of gastrulation and coelom formation in Terebratu- 

 /I'na with. Kowalevsky's observations on Cistella and other brachio- 

 pods reveals many resemblances and some interesting differences. 

 Kowalevsky found that the gastrula was formed by invagination in 

 Cistella, Terebratula and probably Terebratidina ; by delamination 

 or ingression va Thecidium. In all cases, however, he describes 

 the coelom as arising as two lateral pouches from the archenteron in 

 the same manner as in Sagitfa, viz., by the folding into the arch- 

 enteron of two lateral partitions. In this way the archenteron is 

 divided into three portions, a median one which becomes the 



PROC. AMER. PHILOS. SCO. XLI. 168. D. PRINTED MAY 5, 1902. 



