DEVELOPMENT OF TEE FORMS OF ANIMAL LIFE. 



491 



quently differs in its general phenomena. The 

 segmentation is mainly restricted in the meroblas- 

 tic ova of birds to the germinal disk or cieatricu- 

 la, and does not immediately involve any part of 

 the larger remainder of the yolk. This takes 

 place during the time of the descent of the yolk 

 through the oviduct, when the yolk is receiving 

 the covering of the white or albumen, the mem- 

 brane and the shell, previous to being laid— a 

 progress which, in the common domestic fowl, 

 usually occupies less than twenty-four hours. 

 Corresponding essentially to the more complete 

 segmentation of the mammal's ovum, the process 

 leads to the same result in the production of two 

 layers of nucleated formative cells in the original 

 seat of a protoplasmic disk; a bilaminar blasto- 

 derm resulting as in the mammal's ovum, though 

 in a somewhat different relation to the yolk. 



I will not fatigue you with a description of 

 the details of these phenomena, interesting as 

 they may be, but only mention generally that 

 they consist in the formation of deep fissures 

 running from the surface into the substance of 

 the germ-disk. The first of these fissures crosses 

 the disk in a determinate direction, dividing it 

 into two nearly equal semicircular parts. In the 

 next stage another fissure, crossing the first nearly 

 at right angles, produces four angular segments. 

 Then come four intervening radial fissures, which 

 subdivide the four segments into eight; and next 

 afterward the central angles of these eight radial 

 segments are cut off from their peripheral por- 

 tions by a different fissure, which may be com- 

 pared to one of the parallels of latitude on the 

 globe near the pole where the radial or longitude 

 fissures converge. And so thereafter, by the 

 succession and alternation of radial and circular 

 clefts, which, however, as they extend outward, 

 come soon to lose their regularity, the whole 

 germinal disk is divided into the two layers of 

 nucleated cells, constituting the blastoderma or 

 germinal membrane of Pander and all subsequent 

 embryologists. 1 If a laid egg be subjected to 

 the heat of incubation for eight or ten hours, 



1 The more exact nature of the process of segmenta- 

 tion was first made known by the interesting researches 

 of Bagge in 1841, and more especially of Keilikerin 1843. 

 The phenomena of complete segmentation were first fully 

 described in the mammal's ovum in BischofTs " Descrip- 

 tion of the Development of the Rabbit," 1842, and followed 

 out in his succeeding " Memoirs on the Dog, Guinea-pig, 

 and Roe-deer." The phenomena of partial segmentation 

 were first made known, in their more exact form, by Kol- 

 liker's " Researches on the Development of the Cephalo- 

 poda," published in 1S44. In birds the process was first 

 described by Bergmann in 1S46, and more fully by Coste 

 in 1S43. 



the cicatricula, now converted into this seg- 

 mented blastoderm, is found to be considerably 

 expanded by a rapid multiplication of its constitu- 

 ent cells, and in as many more hours, by further 

 changes in its substance, the first lineaments of 

 the chick begin to make their appearance. Simi- 

 lar changes affect the blastoderm of the mammal, 

 and thus it appears that the result of segmenta- 

 tion, in the bird as well as in the mammal and 

 other animals, is the production of an organized 

 laminar substratum, which is the seat of the sub- 

 sequent embryonic development. 



I must still request your attention to some 

 details connected with the process of segmenta- 

 tion, which bear upon the question of the origin 

 of the new cells, and on which recent research 

 has thrown a new and unexpected light. 



With respect to the nature of the first seg- 

 ment-sphere of the ovum, and the source of its 

 nucleus, as well as of the other segment-spheres 

 or cells which follow each other in the successive 

 steps of germ-subdivision, it appears probable 

 from the researches of several independent ob- 

 servers, and more especially of Edward Yan 

 \ Beneden and Oscar Hertwig, that, in the course 

 ' of the extrusion of the germinal vesicle, a small 

 portion of it remains behind in the form of a 

 minute mass of hyaline substance, to which Van 

 Beneden has given the name. of pronucleus, and 

 that, as the result of the fertilizing process, there 

 is formed a second similar hyaline globule or 

 pronucleus, situated near the surface, which grad- 

 ually travels toward the centre and unites with 

 the first pronucleus, and that these two pronuclei, 

 being fused together, form the true nucleus of 

 the first segment-sphere. According to this view 

 the original germinal vesicle, when it disappears, 

 or is lost to sight, as described by so many em- 

 bryologists, is not dissipated, but only under- 

 goes changes leading to the formation of the new 

 and more highly-endowed nucleus of the first 

 embryonic or segmental sphere. It further ap- 

 pears that the subdivision of each segmenting 

 mass is preceded by a change and division of the 

 nucleus, and that this division of the nucleus is 

 accompanied by the peculiar phenomenon of a 

 double conical or spindle-shaped radial lineation 

 of the protoplasm, which, if we were inclined to 

 speculate as to its nature, seems almost as if it 

 marked out the lines of molecular force acting in 

 the organizing process. These lines, however, it 

 will be understood, if visible with the microscope, 

 even of the highest magnifying power yet attained, 

 belong to much larger particles than those of the 

 supposed molecules of the physicist; but con- 



