38 NOTES, ETC., OX THE AUSTRALIAN ROCK OYSTER. 



a central clear area with a contained nodular structure, the two 

 representing what are distinguished technically by the titles of 

 the ' germinal vesicle ' and ' germinal 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 protruberance 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. 'J^'he 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, superim- 

 posed 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, dis- 

 tinguished as the formative cells. The larger basal cell, on the 

 other hand, represents the nutritive or vegetable element, out of 

 which will be constructed the stomach, alimentary track, and all 

 the appended viscera. 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 track. Fine hair- 

 like cilia are at this stage developed upon the external surface of 

 the embryo, and by means of which 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 pro- 

 gress more slowly. From the tenth to about the fifteenth hour 

 the gelieral 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 common to some period in the 



