cess of fecundation, or the phenomena which 

 follow it, but it may be proper to state here 

 in a general way the relation which subsists 

 between the earliest changes occurring after 

 fecundation, and the commencement of those 

 phenomena of a histogenetic nature which 

 precede the formation of the embryo itself. 

 The most obvious and constant of these 

 changes is that known as the cleavage of 

 segmentation of the yolk, a process which 

 has been observed in the ova of all animals, 

 and is not less interesting from its own na- 

 ture, than from the bearing of its phenomena 

 upon the explanation of the earliest organising 

 process of embryonic development, and upon 

 the whole subject of histogenesis. 



The segmentation affects only that part of 

 the ovum of animals which is directly ger- 

 minal or formative ; and it results in the pro* 

 duction of that layer of organised cells, of 

 variable extent, in the centre of which, in a 

 determinate position and direction, the rudi- 

 ments of the embryo are first formed. The 

 process of segmentation is, therefore, the pre- 

 lude to the formation of the Blastoderma or 

 germinal membrane of Pander. 



The extent, therefore, to which segmenta- 

 tion affects the yolk differs greatly according 

 to the amount of the yolk-substance which is 

 directly germinal ; that being in some animals 

 the whole, and in others only a fraction of 

 the yolk, in proportion to the part which is 

 only indirectly nutritive. In that group of 

 ova, then, to which those of Mammalia belong, 

 and which we have called the small-yolked, 

 the entire yolk, or, at all events, its superficial 

 layer, being directly formative, or being in- 

 volved from the first in the production of the 

 Blastoderm, the segmentation is complete, or 

 the process of cleavage affects the whole mass 

 of the finely granular yolk within the 2ona or 

 vitelline membrane. In those ova again, such 

 as we find in the bird among vertebrate, and 

 the cuttlefish among the invertebrate animals, 

 in which the formative yolk has the most 

 limited extent, and consists only of a finely 

 granular disc near the surface of the much 

 larger mass of the cellular nutritive yolk, the 

 segmentation is confined to that disc alone, 

 and is therefore, in some respects, widely 

 different from that which occurs in Mam- 

 malia. In the intermediate group of ova be- 

 longing to Batrachia and Osseous Fishes, there 

 are many gradations of transition from the 

 complete to the partial cleavage, so that in 

 some, as the common frog, it is nearly, but 

 not entirely, over the whole yolk ; while in 

 others, as in the salmon or osseous fishes, it 

 does not extend over more than a third of 

 the surface of the yolk.* 



* The more important phenomena of the yolk- 

 germ cleavage or segmentation have been ascer- 

 tained principally by the following observations: 

 viz. 1st. of Prevost and Dumas in Batrachia, as 

 early as the year 1824, and subsequently of Rusconi 

 and Von Baer in the same animal ; 2nd. of Bischoff 

 and Barrj- in Mammalia, in 1838-39, their obser- 

 vations being confirmed by myself in 1810, and 

 greatly extended by Bischoff before the publication 

 of his work upon the development of the rabbit, in 

 1842 j 3rd. of Bagge in 1841, and of Kolliker in 



OVUM. [139] 



In the greater number of instances there is 

 recognised in the mass of the whole yolk, or 

 in its germinal part, immediately previous to 

 its undergoing segmentation, a clear simple 

 cell, generally nucleated, which was not be- 

 fore perceptible ; to this the name of embryo- 

 cell has been given, in order to distinguish it 

 from the germinal vesicle, from which it has 

 hitherto been believed it is in some measure 

 distinct. In other instances a clear spherule 

 or space only is observed in the place of the 

 embryo -cell, and in a few animals no clear 

 part of this nature has yet been detected. 

 The division of the embryo-cell accompanies, 

 or rather immediately precedes, that of the 

 germ-yolk, so that each mass formed by the 

 cleavage, grooving, or segmentation, as the 

 case may be, contains as its nucleus or centre 

 an embryo-cell, or clear spherule of its own, 

 descended from the first cell or spherule of 

 the same description. 



The process of segmentation, whether it 

 involves the whole ovum, or is limited to a 

 larger or smaller disc of the yolk, proceeds 

 in most animals with a certain degree of geo- 

 metric regularity, so that the number of germ- 

 yolk segments are successively multiplied so 

 as to be in the numbers 2, 4, 8, 16, 32, 64, 

 &c., until by the ultimate division a vast 

 number of small globular masses are formed, 

 which occupy principally the surface of the 

 yolk over all its germinal portion.* 



The last result of the segmentation is the 

 production of the blastoderma or germinal 

 membrane in which, by other changes, the 

 rudiments of the embryo subsequently make 



1843, on Xematoid Worms ; 4th. of C. Vogt in the 

 Salmonidae and in the Alytes Obstetricans in 1842 ; 

 5th. of the same author, of Quatrefages, and many 

 others in various invertebrate animals ; 6th. in its 

 most limited form the phenomenon was first well 

 described by Kolliker in Cephalopoda in 1844 ; and 

 7th. in birds by Bergmann in 1846, by Coste in 

 1848, and by myself in the following years. The 

 observations on this subject are far too numerous 

 for quotation; those especially which have been 

 made in experiments by artificial fecundation are 

 most favourable to the investigation of the seg- 

 mentation and other phenomena which follow 

 immediately on fecundation. And in all these 

 instances, as well as in very numerous others, the 

 occurrence of segmentation and the regularity of 

 its phenomena are so constant that we may regard 

 it as one of the best established series of facts in 

 organic nature. The observations with regard to 

 segmentation in the ova of insects, which are still 

 imperfect, form the only exception to the foregoing 

 statement with which I am acquainted. 



* Reference is here made chiefly to the best- 

 known and most common kind of segmentation, in 

 which this process consists in the massing of the 

 granular and fluid substance of the yolk round the 

 embryo-cells or clear spheres as centres ; but it is 

 right to state that there is another form of this 

 process, as yet only observed in some of the Cestoid 

 and Nematoid Entozoa, in which the yolk, either 

 clear or granular in its structure, does not appear 

 to follow the divisions of the embryo-cells, but the 

 gradually increasing progeny of the latter assi- 

 milate or combine more and more with the yolk, 

 so that at last, when the germinal part of the ovum 

 is entirely occupied with new cells, the original 

 yolk has quite disappeared. The nature of this 

 process, as compared with the more common form 

 of yolk segmentation, is not perhaps as yet fully 

 understood. 



