DEVKLOl'MENT OF FISHES. 297 



(" Lectures on Invertebrata," p. 77.), the correspondence will be seen 

 to be extremely close. In the Entozoon the entire yolk is the seat of 

 the successive subdivision produced by reiterated processes of deve- 

 lopment, liquefaction, and assimilation, of nucleated cells, until the 

 property of the primary impregnated germ-cell has been distributed 

 throughout the mass ; when the whole mass, by subsequent meta- 

 morphosis of the cells, is converted into the embryo. In the fish a 

 part only of the yolk is the seat of these processes, superficially indi- 

 cated by fissures of that part, and the resulting embryo is connected 

 with the remainder of the yolk ; this remainder is called the nutrient 

 yolk, or ' vitellicle;' the other part is the 'germinal yolk,' sometimes 

 called the ' germinal membrane,' from its being thinly spread over 

 more or less of the nutrient yolk before the embryo arises out of it. 

 In the Tench and other Cyprinoids it overspreads the whole nutrient 

 yolk*: in the Blenny, Rathkef found the embryo considerably ad- 

 vanced before the nutrient yolk was so included. The surface of the 

 so-covered yolk is ciliated, and, with the embryonal part, performs a 

 slow but regular rotation within the albuminous fluid of the chorion. | 

 The first traces of the embryo are two parallel ridges, the ' laminre 

 dorsales,' which coalesce, and form the neural axis and the rudiments 

 of the chorda dorsalis {^fig- 78./"). The germinal membrane separates 

 into an outer ' vertebral' and an inner ' visceral' layer : from the 

 foi'mer are developed the brain and myelon, the vertebra) and their 

 appendages, the muscles and nerves, and the skin ; from the latter 

 are developed the digestive, excretory, and generative viscera. The 

 outer layer has been called the 'serous' and 'animal' layer; the 

 inner one the ' mucous' and ' organic' layer. Between these are deve- 

 loped the organs of circulation and respiration. The ' lamintc dor- 

 sales' consist of the extension of the vertebral layer upwards (the 

 embryo being supposed to be prone) to inclose the neural axis : tlie 

 ' lamina) ventrales' are downward extensions of the same layer, to 

 inclose the viscera and the nutrient yolk ; consequently the so- 

 extended ' lamina) ventrales,' when they coalesce below, form the 

 external (serous, or more properly tegumentary) layer of the yolk- 

 sac. After the trunk is developed, the head and tlie tail appear, and 

 project freely from the supporting surface, and tlie embryo encircles 

 the yolk, in the form of an apodal fish {th. g). With regard to the 

 skeleton, the aponeurotic septa of the vertebral segments of the body 

 first appear ; then the ' chorda dorsalis ;' afterwards the rudiments of 

 the neurapophyses along the sides of the neural axis ; and, lastly, the 

 hfemal arches and their appendages. At this time may be discerned 

 the characteristic strias of the muscular fibre. The development of 

 the skull is described at p. 71. 



* xLiu. (1831). t txxviii. \ cxxxii. 



