1902.] CONKLIN — EMBRYOLOGY OF A BRACHIOPOD. 53 



elongation antero-posteriorly have already been described. At the 

 same time the embryo becomes wider at its anterior end and nar- 

 rower posteriorly. The dorsal mantle groove and fold appear and 

 the fold extends over on to the ventral side of the larva as a slight 

 swelling on each side of the midline (Figs. 25 and 26). These 

 are the halves of the ventral mantle fold and at first they are sepa- 

 rated in the mid-ventral line by the blastopore groove (Fig. 26, Bf)^ 

 while the blastopore remnant {^Br) lies at the anterior end of this 

 groove. Very soon after this stage the halves of the ventral mantle 

 fold fuse with each other, thus obliterating the last trace of the 

 blastopore groove, while a deep notch on the anterior side of the 

 ventral mantle fold (Fig. 29, O) represents the place at which the 

 blastopore remnant was last seen. 



I. The Mafitle J*old is at this stage a ring-Hke prominence which 

 extends all the way around the larva (Figs. 29, 30, 31). This ring 

 passes obliquely around the larva, being nearer the anterior end 

 on the dorsal side and nearer the posterior on the ventral side 

 (Fig. 31). Two mantle furrows are now plainly distinguishable, 

 one in front of and the other behind the mantle fold. The anterior 

 furrow is deepest on the ventral side, while the posterior one is 

 deepest on the dorsal side (Figs. 29-31), With the appearance of 

 the mantle, bounded anteriorly and posteriorly by these constric- 

 tions, the mantle furrows, three regions may be recognized in the 

 larva, viz., the cephalic region, in front of the anterior mantle 

 furrow, the mantle, between the anterior and posterior furrows, 

 and the peduncular region, behind the posterior mantle furrow. 



These constrictions, which I have called the anterior and pos- 

 terior mantle furrows, continue to grow deeper but at no time do 

 they form true septa which divide the ccelom. The regions which 

 they separate are not, therefore, segments, as Kowalevsky supposed. 

 The larva is at this, and all other stages which I have studied, 

 unsegmented, and the appearance of segmentation is due merely 

 to the formation of the mantle from the middle region of the 

 body. 



The mantle becomes a very prominent ring around the body, and 

 then its free edge is turned backwards until it surrounds the pedun- 

 cular regions on all sides (Figs. 32-36). A space is left between 

 the mantle and the peduncle which is the peduncular chamber 

 (Figs. 34 and 36, PC). This chamber is a little deeper and wider 

 on the dorsal than on the ventral side, which is due to the fact 



