562 BEPORT OF COMMISSIONER OF FISH AND FISHERIES. [108] 



"The nucleus is iieitbcr inevitably present in tbe cell nor yet in ]>ro- 

 toplasui. Plasmodium on the one liand, and enuclear protoplasm on 

 the other, prove this. 



" ihe surface growth in extent of cellular raemhranes is conditioned 

 as a rule by two cleavage planes of the cells, perpendicular or at right 

 angles to each other. Attending growth in thickness, as in the epider- 

 mis, for example, there is, besides the above, a cleavage plane of the 

 cells parallel to the surface." 



The laws of growth and cleavage, \vhich are touched upon in the fore- 

 going extracts, are to some extent a realization of what must have 

 coursed through the mind of almost every student who has busied 

 himself with embryological investigations, or who has watched the 

 recent advances of histology without himself taking an active part in 

 the work. For my own part I believe that we have arrived at a new 

 era in embrj'ological and physiological research. We shall hereafter 

 not only be obliged to figure and study the changes of external form 

 which transpire during development, and the contours of the cells which 

 are concerned in bringing this about in an embryo, but also the phe- 

 nomena manifested and the changes suffered by the whole contents of 

 the individual cells in the process of embryonic evolution. Here is 

 where anatomy and physiology converge; and upon a comparative em- 

 bryology as exhaustive in its methods and results as that here contem- 

 plated, will it alone be possible to found a comparative physiology 

 equally" exhaustive, but infinitely more valuable in its practical appli- 

 cation to the needs of every-day life than the physiology of the present. 

 The masterly monographs of Strasburger, Flemming, Fol, the Hertwigs, 

 Whitman, Leidy, and Mark, besides many others, have brought us face 

 to face with a series of facts and phenomena, the significance of which 

 has hardly yet been full}' apjireciated. 



27. — The gastrula and cceloma of teleosts. 



The epibolic growth of the blastoderm over the yelk of the osseous 

 fish ovum, as in other meroblastic ova, has given rise to not a little 

 discussion amongst embryologists as to the true nature of its gastrula 

 stage. As Whitman * has pointed out, there is a fundamental similarity 

 in the mode of formation of the neurula in embryos of Glepsine, the frog, 

 sturgeon, salmon, chick, and saw-fish. The concrescence of the rim of 

 the blastoderm to form the neurula seems in reality to be the key to 

 the interpretation of the development of the gastrula of the embryos of 

 bony fishes and some other meroblastic types, as well as the develop- 

 ment of the cceloma and lateral musculature of the body. 



In order to make it easier to understand the gastrula of Teleostei, a 

 series of diagrammatic and semidiagrammatic figures are introduced 

 here. 



^Embryology of Clepsine, pp. 86-05. 



