ADDRESS. l.XXKV 



is mainly restricted in the meroblastic ova of birds to the germinal disk or 

 cicatricula, and does not immediately involve any part of the larger re- 

 mainder 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 membrane, and the shell, previous to being laid — a 

 process -which, in the common domestic fowl, usually occupies less than 

 twenty-four hours. Corresponding essentially to the more complete segmen- 

 tation 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 bilamiuar blastoderm 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 with rounded edges 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 afterwards the central 

 angles of these eight radial segments are cut off from their peripheral portions 

 by a different fissure, which may be compared 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 outwards, 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 

 subsequent embryologists *. If a laid egg be subjected to the heat of 

 incubation for eight or ten hours, the cicatricula, now converted into this 

 segmented blastoderm, is found to be considerably expanded by a rapid 

 multiplication of its constituent cells ; and in as many more hours, by further 

 changes in its substance, the first lineaments of the chick begin to make their 

 appearance. Similar changes affect the blastoderm of the mammal ; and thus 

 it appears that the result of segmentation, 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 subsequent embryonic development. 



I must still request your attention to some details connected with the 

 process of segmentation, which bear upon the question of the origin of the 



* The more exact nature of the process of segmentation was first made known by the 

 inli resting researches of Bagge in 1841, and more especially of Kolliker in 1843. The phe- 

 nomena of complete segmentation were first fully described in the mammal's ovum in 

 Bischoff's description of the development of the Babbit, 1842, and followed out in his 

 succeeding memoirs on the Dog, Guineapig, and Eoedeer. The phenomena of partial 

 segmentation were first made known, in their more exact form, by Kolliker's researches 

 on the development of the Cephalopoda, published in 1844. In birds the process was 

 first described by Bergmann in 1846, and more fully by Coste in 1848. 

 1877. a 



