2 Mr. E. E. Prince on the Significance of the 



Kingsley and Conn, and others) have adopted this view, 

 according to which the egg of an osseous fish is, perliaps, one 

 of the most marked examples of the raeroblastic type. 



In the Mammalian ovum we know that there is no such 

 broad distinction ; but, as in Amphioxus, the yolk that is 

 present and the active protoplasm are so intermingled that 

 segmentation is complete. The Amphibian ovum — Rana, for 

 example, is also holoblastic ; but the yolk so preponderates 

 towards the vegetal pole that the cleavage-furrows, beginning 

 at the opposite or animal pole, progress with increasing diffi- 

 culty as they approach the former region. The animal pole 

 in the Amphibian egg is distinguished by the great abun- 

 dance of active protoplasm and the minute size of the sus- 

 pended yolk-spherules, as well as its more rapid cleavage. 

 Still more marked is this bipolar segregation in the Sauropsidan 

 and Elasmobranch ovum ; but in the Teleostean egg it is 

 most complete — a distinctly marked germinal disk, composed 

 almost entirely of clear protoplasm, being formed by the with- 

 drawal of germuial matter from the granular yolk. The 

 separation may be very apparent, even before fertilization, in 

 certain Teleosteans — a discus proligerus collecting, similar to 

 the superficial protoplasmic disk seen lying upon the yellow 

 food-yolk in the mature Selachian ovum. 



Usually both constituents are so intermingled as to be undis- 

 tinguishable in the living egg until a period of one or two 

 hours has elapsed after the entrance of the spermatozoon, 

 when the translucent homogeneous blastodisk is rapidly out- 

 lined at the animal pole, either at the upper or the lower side 

 of the egg, according to the species *. The separation of 

 germinal matter from the food-yolk is carried to such a degree 

 in the Teleostean ovum that it presents a marked contrast to 

 the type of egg seen in the bird or shark, and still more in 

 the frog or lamprey (compare figs. 1 and 2, PL II.). E. van 

 Beneden, in his classical memoir " Sur la composition et la 

 signification de I'oeuf ^' f, speaks of the nutritive part as 

 deutoplasm, and lays stress on its non-integral or accessory 

 nature, on its purely passive function, and on the fact that in 

 some eggs it is absent, though when it is present it serves to 

 nourish the blastoderm and embryo. This contrast between 

 the deutoplasm and the germinal protoplasm is illustrated in a 

 marked degree in the Teleostean ovum, yet the existence in it 



* In the Salmonidse the germ surmounts the upper pole of the e^g, 

 whereas in the ova of the Pleuronectidse and Gadidse it is formed at the 

 inferior pole. 



t E. van Beneden, M^m. Coar. I'Acad. Roj. de Belgique, tome xxxiv. 

 1870. 



