276 



SCIENCE. 



[Vol. I., No. 10. 



the axis or diameter of the egg, and not at 

 right angles to it. This is shown in the dia- 

 gram (fig. 4) , in whicli the first segmentation- 

 nucleus has l)een metamorphosed into the 

 cleavage spindle (s/j), with the upper end em- 

 bedded in the germ-disk (p) , and the lower 

 end embedded in the protoplasmic layer (p'), 

 which consists of the protoplasmic matter not' 

 incorporated into the germ, and left over to 

 cover the deutoplasm (d). (The thickness of 

 the layer (p') has been exaggerated in the fig- 

 ure for the sake of clearness.) When the spin- 

 dle {sp) has separated equatoriallj-, leaving its 

 upper end in the germ as its nucleus, and its 

 lower end in the protoplasmic layer covering 

 the .yelk, as the parent of the free nuclei which 

 afterwards appear in that layer, we may saj- 

 that the true first segmentation has occurred, 

 which has separated the deutoplasmic or yelk 

 pole of the egg as a single cell from the germ- 

 cell. We see, therefore, that the amount of 

 deutopliism in excess of the germinal matter 

 actually determines the plane of separation 

 between the germ and the yelk. We can also 

 understand how such an arrangement would 

 cause the mode of development to be modified. 

 The meroblastic and centrolecithal types of 

 ova, on account of the preponderance in bulk 

 of the j-elk-mass, are compelled to develop 

 the blastoderm from the disk bj' spreading, or 

 epibole, or bj' simultaneous superficial delam- 

 ination, and cannot be directly transformed 

 into a hollow blastula, as in a holoblastic 

 ovum. 



The consequences of the displacement of the 

 nucleus are therefore of great significance in 

 embryology ; but the adaptations resulting from 

 the permanent displacement of the nucleus of 

 the ovicell during its development do not end 

 with what has been said in the preceding par- 

 agraph. The layer p', of fig. 4, acquires an 

 important physiological function in conjunction 

 with the blood-vascular system, in that it be- 

 comes an organ for breaking down and elabo- 

 rating the yellv into blood-cells in fish ova, as 

 shown by the researches of Vogt, Kupffer, 

 Gensch, Hoffmann, and myself. From the 

 remarkable similaritj- of the mode of develop- 

 ment of the eggs of elasmobranchs, reptiles, 

 and birds, to that of the osseous fishes, — in re- 

 spect to the mode of germ-formation, spreading 

 of the blastoderm, and the development of free 

 nuclei, in the former and latter tj'pes at least, — 

 I should not be surprised if it would be yet 

 determined that such a structure exists in the 

 ova of all of them. 



The occurrence of free nuclei, under the 

 blastoderm of the ova of Loligo, Sepia, and 



Octopus, embedded in the yelk, as found by 

 Lankester ; in arthropods b\' different ob- 

 servers ; in those of osseous fishes by Kupffer, 

 Gotte, Oellacher, His, Klein, Ziegler, Gensch, 

 Hoffmann, Rauber, and myself; in the ova of 

 sharks bj' Balfour and Schultz ; in those of 

 birds by Gotte, Rauber, and Balfour, — is 

 stronglj' in favor of the doctrine that thej' have 

 a similar function throughout all of these vari- 

 ous forms. Their origin is, doubtless, not 

 spontaneous, as has been believed bj' some ; 

 but, like the nuclei of the blastoderm itself, they 

 have been primarily derived from the first seg- 

 mentation-nucleus. In Clepsine, according to 

 Whitman, it appears that thej^ enter into 

 the formation of the hypoblast. 



Furthermore, it is probable that the develop- 

 ment of the germ is actually to be viewed as 

 a process of growth, — concentration of tlie 

 germinal matter at the animal pole in virtue 

 of its own power of movement. Finallj^ I 

 would regard the deutoplasm as so much stored 

 material, which — just as the fat globules in a fat 

 cell have pushed the nucleus to the periphery, 

 or as the accumulating fluids in the chorda cells, 

 or as the enlarging sap-cavity in a plant-cell — 

 has displaced the nucleus, and made it assume 

 a parietal position. In evidence of this, I 

 would cite the oval, flattened globoids of the 

 deutoplasm of Lepidosteus (ichthine of Valen- 

 ciennes and Fr6my) as analogous to the stored 

 proteids in many plant-cells. The frequent 

 considerable displacement of the nucleus from 

 the centre of the body in Amoeba, on account 

 of the presence of great numbers of food-vac- 

 noles in the endosarc, seems to be a phenome- 

 non of a similar nature. 



The rather anomalous segmentation of the 

 eggs of the frog, lamprey, and Clepsine^ must 

 be noticed here, as they would appear to form 

 an exception to the principle for which we have 

 contended in truly meroblastic ova; viz., the 

 final dissociation of germinal and deutoplasmic 

 matter at the time of the first cleavage, which 

 divides the whole egg into two nearly equal 

 blastomeres. Immediatel.y or verj^ soon after 

 the first cleavage, the segmentation again 

 becomes unequal, in that smaller blastomeres 

 are formed at the pole where the polar cells have 

 been, or may be supposed to have been, ex- 

 truded. In this way, it results that a certain 

 mass of cells at the germinal pole of the ovum 

 divide much more rapidly than those containing 

 more deutoplasm at the opposite pole. Now, 

 it is singular that in these tj'pes we actuall}' 

 have an approximation towards the develop- 



1 Whitman, Embryology of Clepsine, — ( Quart, jovrii. micr. 

 sc, July, 1878.) 



