On the Embryology/ tfcc. of Yoldia limatula, Say. 267 



XLIV. — Notes on the Embryology, Anatomy, and Habits of 

 Yoldia limatula, Say. By GiLMAN A. Drew ^. 



In this sliort abstract attention will briefly be called to some 

 points of interest in the development, anatomy, and habits of 

 Yoldia limatula, a member of Pelsner's order Protobranchia. 

 A more detailed description will follow at a later time. 



Embeyology. 



The eggs are about '15 millim. in diameter, of a chocolate- 

 brown colour, and very opaque. They are laid free in the 

 water, and are not encumbered by any kind of envelope. 

 The polar bodies are lost soon after they are formed. The 

 first cleavage results in the formation of subequal blastomeres. 

 Subsequent divisions give rise to an epibolic gastrula in 

 which there may be as many as sixty outer ectoderm-cells. 

 Some of the ectoderm-cells wander into the interior, the ento- 

 derm-cells divide, and at one side of the resulting cell-mass a 

 narrow tube appears, which opens to tlie exterior through the 

 blastopore. As it has not been determined whetlier this tube 

 is ectodermal or endodermal in origin, whether it represents a 

 stomodffium or a primitive gut, it will throughout this descrip- 

 tion be referred to as the ventral tube. 



While these changes are taking place about forty-two of 

 the surface ectoderm-cells enlarge, become vacuolated, and 

 acquire cilia. The embryo also elongates (fig. 1), becomes 

 cylindrical, acquires an apical plate which bears a tuft of 

 apical cilia, ac, and the outer cells, which will hereafter be 

 referred to as test-cells, arrange themselves in five rows. Tlie 

 blastopore, bl, occupies the end of the cylinder opposite the 

 apical plate, and a depression, x, appears near the anterior 

 end of what may now be referred to as the ventral side. The 

 two end rows of test-cells are entirely covered with fine cilia, 

 and the three intermediate rows each bear a band of much 

 longer cilia. 1 he embryo now swims in more or less definite 

 lines, rotating the while upon its longitudinal axis. During 

 activity the apical cilia are bunched together into a sort of 

 whip, and always precede the embryo. 



Inside the test a new ectoderm is formed, probably from the 

 ectoderm-cells tiiat wander in. At the age of about forty 

 hours a few of tiiese ectoderm-cells elongate and give rise to 

 the shell-gland, which spreads over the dorsal portion of the 



* From the 'Johns Hopkins Unhersity Chculars,' November 18U7, 

 pp. 11-14. 



