OYSTERS AND OYSTER FISHERIES OF QUEENSLAND. 275 



ovum. The fusion between the two elements that then takes place is not easy to trace, but the 

 results arising from the union are speedily manifested. The ovum, prior to fertilisation, was 

 distinguished by the presence of a central clear area with a contained nodular structure, the two 

 representing what are distinguished technically by the titles of the "germinal vesicle" and "ger- 

 minal spot," or the "nucleus" and "nucleolus." Shortly after fertilisation, the substance of the 

 ovum becomes opaquely granular throughout, and the germinal vesicle is no longer visible. 

 Within the second hour, a small globular protuberance will have made its appearance at the broader 

 end of the ovum, and opposite to the micropyle. This is the so-called directive, or polar, cell. 

 Quickly following upon this, the entire body-mass of the ovum becomes furrowed, or constricted, 

 across the centre, and each half is seen to contain a central nucleus. The upper half, associated 

 with the polar cell, now divides itself into two equal parts. These again split into four and next 

 into eight, the aspect of the ovum or embryo, as it may now be correctly termed, at about the end 

 of the third hour being that of a number of small coherent cells, superimposed symmetrically on 

 the top of a large basal cell. 



This condition of development represents an important phase in the life-history of the embryo 

 oyster. There are now present all the essential elements out of which the perfect animal will be 

 built up. Out of the smaller superincumbent cells all the investing membranes, tactile organs, and 

 essential animal structures will be fashioned, and they are consequently distinguished as the 

 formative cells. The large basal cell, on the other hand, represents the nutritive or vegetative 

 element, out of which will be constructed the stomach, the alimentary tract, "and its appended 

 glands." Within from four to six hours, the smaller, or formative, cells have so increased and spread 

 as to completely enclose the large nutritive cell, and which in its turn now divides up and lays the 

 foundation of the alimentary tract. Fine hair-like cilia are at this stage developed upon the 

 external surface of the embryo, and by means of these it progresses through the water in an 

 irregular rotatory manner. The polar cell, which up to this stage had occupied a conspicuous 

 position, now breaks loose and disappears. The metamorphoses from this point progress more 

 slowly. From the tenth to about the fifteenth hour the general shape of the embryo is somewhat 

 kidney, or turban, shaped, it having a slight depression on one side. This represents what is 

 known to biologists as the gastrula stage, a structural phase which has been found to be repre- 

 sented in some period of the development history of almost every known form of animal life higher 

 than the unicellular protozoa. In its most typical condition, this gastrula embryo consists of a 

 cup-shaped body composed of two single cell layers, the outer one being built up of the animal or 

 formative cells, and the inner one out of the nutritive or vegetative cells. The distinctive appella- 

 tions of the " epiblast " and "hypoblast" are commonly applied by biologists to these outer and 

 inner cell layers. 



After passing the "gastrula" stage, development towards the typical organisation of the parent 



oyster proceeds apace. The central cavity representing the stomach opens out by an anterior and 



*M M 2 



