24 COMMISSION OF CONSERVATION 



4:4o p.m., temperature 20.5° C. ( = 68.9°F.) and salinity l.Olo. Many 

 such fertilization and culture experiments were performed at Shediac, Bay 

 du Vin, Caraquet (southern portion of Chaleur bay), and Malpeque (Rich- 

 mond bay, P.E.I.) , between June 26 and September 3. It should not be 

 inferred that the difference in time of development to the same point in 

 the two cases given was due entirely to the difference in temperature at 

 these two periods, and not at all to the difference in salinity. Besides an 

 individuality in the oysters and their early or late ripening, a consideration 

 of importance arises from the artificial method. In neither case were the 

 eggs spawned by a natural impulse on the part of the oysters, which leaves 

 a greater chance that they were not equally mature to begin with. Brooks 

 mentions various times (2, 2^, 2 to 4, 6, 11^ hrs., etc.) and so does Nelson, 

 for the development to the swimming stage. 



The size of the youngest swimming larva (Plate I, fig. 9) is about 

 .062 X .055 mm. (Oc. 5, obj. 4 = 9 x 8 = 9 x 6.9 /^ and 8 x 6.9 fx.) in 

 length and depth. 



The first swimming movement is due to the activity of cilia, de- 

 veloped on the outer surface of the ectoderm cells. These little hair-like 

 processes flap energetically in one direction and too rapidly to be ob- 

 served except in specimens that are injured or dying. At this period the 

 larva is inclined to be rather broader at one end than the other, the broader 

 end being the one at which the polar bodies are situated, if they have not 

 already dropped off. The cilia at this end soon become larger and 

 longer than the rest and stand on the deep cells of a projecting disk, to- 

 gether forming a definite swimming organ, the trochal disk or prototroch, 

 and this stage of the larva is called a trochophore (trochosphere). 



Concurrently with the changes in form and surface, the development of 

 a special organ of locomotion, and the definite marking out of the anterior 

 end of the larva, there is a corresponding process in the invaginated endo- 

 derm. This will ultimately give rise to the epithelium of the intestinal 

 system and its appended organs, but, due to the mode of origin of the endo- 

 derm, it is continuous at the edge of the blastopore with the ectoderm, 

 which contributes in forming the mouth parts. 



Behind the archenteron (Plate V, fig. 21), in the angle of the blasto- 

 coele posterior to the blastopore, there originates what is to become a 

 third layer of cells, the mesoderm (mesoblast) , destined to form the greater 

 bulk of the soft parts of the animal, the muscular and connective tissues, 

 heart, blood, lymph, and other organs. It seems to spring more from the 

 endoderm than from the ectoderm, although at the line of contact between 

 the two, and to spread throughout the blastocoele, lining the ectoderm and 

 coating the endoderm. Exact, conclusive observations of its origin for 

 the oyster are lacking, but the posterior small cells of the endoderm in 

 Brooks' fig. 30 may be homologous with the mesoderm cells in Horst's figs. 



