BY W, SAVILLE-KENT, F.L.S., F.Z.S., ETC. 39 



development history of almost every known form of animal life 

 higher than the unicellular protozoa. In its most typical con- 

 dition this gastrula embryo consists of a cup-shaped body com- 

 posed of two single cell layers, the outer being built up of the 

 animal or formative cells, and the inner one out of the nutritive 

 or vegetable cells. The distinctive appellations of the ' epiblast ' 

 and ' hypoblast ' are more commonly applied by biologists to 

 these respective outer and inner cell layers. 



After passing the ' gastrula ' stage, development toAvards 

 tlie typical organisation of the parent oyster proceeds apace. 

 The central cavity representing the stomach opens out by 

 an anterior and a posterior passage and apertures, which 

 correspond respectively with the throat and mouth, and the 

 intestine and vent. The shells make their appearance at a 

 depression in the dorsal surface, and gradually increase in size 

 until they enclose the entire body. Simultaneously with these 

 metamorphoses a disc covered with powerful vil)ratile cilia has 

 developed at the anterior extremity, and with the assistance of 

 which the embryo oyster can propel itself vigorously through 

 the water. As the shells grow larger and heavier the little 

 oyster becomes less capable of sustaining itself in the water, and 

 finally sinks to the bottom. This is a crucial epoch in the 

 mollusc's existence. Should it settle upon a rock, shell, or 

 other clean, hard substance, it attaches itself to it, and its life 

 is assured ; but should it, on the contrary, light upon soft mud, 

 sand, or other material to which it cannot adhere, it inevitably 

 perishes. The proportion of yoimg oysters that find a secure 

 anchorage in comparison with the vast numbers that are 

 devoured, or become literally lost at sea, is necessarily infini- 

 tesimal. 



The time taken by the embryo of the Australian oyster to 

 pass through the series of metamorphoses enumerated, and to 

 arrive at the attached or sedentary state, has been found by 

 me, under favourable conditions, to average four days, two out 

 these elapsing before the shells become conspicuously apparent. 

 Permanent preparations of the ova and embryos in their various 

 phases of development may be satisfactorily obtained by treat- 

 ment, first with a 1 per cent, solution of osmic acid, and sub- 

 sequent transfer to dilute glycerine, in the proportion of one- 

 half glycerine and one-half-water. Several slides illustrating 



