I9 2 BULLETIN OF THE BUREAU OF FISHERIES. 



Figure 1836 (p. 193) shows an embryo of this same stage that was placed in a dilute 

 solution of glycerine and water. The solution was not strong enough to destroy the 

 organism through osmotic action, but served to stupefy and render it more transparent. 

 The invagination process of gastrulation was occurring as represented, and mesodermal 

 cells were scattered throughout the segmentation cavity. A typical invagination 

 gastrula is formed in the mussel, therefore, as in the oyster, after first starting as an 

 epibolic gastrula. 



Observations on the living eggs during cleavage and formation of the germ layers 

 would lead to the conclusions that the ectoderm arises from the cleavage of the first 

 micromeres formed, the entoderm from an invagination of the micromeres that come 

 to lie in the region of the vegetative pole, and the mesoderm from the macromere that is 

 ultimately surrounded by the micromeral cells. 



DEVELOPMENT OF THE TROCHOPHORE LARVA. 



When the developing Mytilus larva has reached the age of about 20 hours, it begins 

 to enter on a stage very characteristic of the free-swimming larvae of the Lamelli- 

 branchia and which so closely resembles the trochophore larva of the Annelida that it 

 has been designated by the same name. The cilia, already weakly developed over the 

 entire surface of the larva, become very prominent. The body gradually elongates and 

 at the anterior pole, just in front of the mouth, a zone of very large cilia is formed which 

 encircles the apical plate. The similarity to the annelidan trochophore is still further 

 emphasized by the flexible flagellum that protrudes from the center of the apical plate, 

 as shown in figure of embryo 42 hours old (fig. 183(f). At this stage the larvae become 

 very active swimmers. The flagellum is carried forward and appears to serve as a 

 tactile organ, while the body cilia beat with the effective stroke backward in such a way 

 as to drive the larva forward with a spiral, clockwise motion. 



The fate of the blastopore could not be determined. It apparently disappears in 

 the region later occupied by the proctadeum. At about the fortieth hour of develop- 

 ment the digestive tract appears in well-defined form (fig. 183c?). The stomadeum 

 arises as an invagination of the ectoderm just behind the apical plate and is lined with 

 well-developed cilia which beat with the effective stroke inward. It connects with the 

 anterior end of the stomach. The stomach, when it first appears, has an oval form 

 and is lined with large cells. At the same time the stomadeum is formed the procta- 

 deum arises a short distance behind the mouth and connects with the posterior region 

 of the stomach. 



The shell gland is observed, immediately following the formation of the digestive 

 tract, as a thickened portion of the ectoderm in the posterior dorsal region (fig. i83e, 

 SG). The cells are glandular in character and continue to grow anteriorly until they 

 cover the mid-dorsal line. Two or three hours later the beginning of the embryonic 

 shell is visible as a thin integument over the dorsal surface of the gland (fig. 183/, Sh). 

 When first secreted it is unpaired, but as growth continues over the sides of the larva a 

 median dorsal dividing line is formed which separates the shell into right and left valves 

 (fig. 183I1, Sh). This line of division corresponds to the hinge line of the adult shell. 

 During the next few hours the growth of the shell is the most conspicuous change 

 observed in the larva. At the age of 69 hours the valves are almost large enough to 

 inclose the fleshy parts completely (fig. 1831). 



