THE BLASTODERM. 681 



mentioned topic it may be remarked that certain heaving and rotatory motions 

 which have been observed by several cmbryologists immediately before and dur- 

 ing the occui-rence of the cleavage, indicate the play of contractile and it may 

 be°of other forces within the protoplasm ; and these forces have been supposed to 

 have some relation to the nucleus. Recent observations by Flemming in the 

 ovum of Anodonta, of Oellacher in that of the trout, and of Goette in the toad, 

 seem to show that there is some structm-al condition related to the process of 

 division which may have a connection with its occm-rence ; for in the eggs of 

 these animals the space within the domain of each segment sphere about to be 

 formed is occupied by fine filaments radiating from the centre towards the 

 circumference, and preceding the formation of the clear nuclear space within. 

 It is probable that the hyaline globules may be the result of the first yolk 

 contraction. 



Secondary Segmentation.— The segmenting process previously described 

 may be called j^ >• i ma /■//, for it is not yet ascertained in how far the whole of the 

 blastoderm, considered as the organised substratum for the development of the 

 new being, owes its origm directly to the first process of germ segmentation, 

 or to what extent a later process of an analogous kind may contribute to the 

 formation of some of its deeper elements. The most recent observations, such as 

 those of Oellacher and Goette on the egg of the bird, of Ray Lankester on the 

 ova of Cephalopoda, and of Balfour on that of sharks, would tend to support the 

 view that in meroblastic ova at least, the process of segmentation, considered as 

 one of conversion of the yolk into blastodermic elements, is not completed in 

 the first series of such divisions, but continues to take place in a modified form 

 for some time afterwards, thus extending the blastoderm over the surface of the 

 yolk more and more by the addition of newly acquired elements. These elements 

 appear to be fonned from nev.^ centres of cell organisation external to the limits 

 of the germinal part of the ovum, by what may be called a process of free cell 

 formation, and to contribute mainly to the production of the deeper part of the 

 blastoderm. (Ray Lankester in Ann. & Mag. of Nat. Hist., 1873, p. 81, and F.M. 

 Balfour in Jom-n. of Microscop. Science, July, 1873 and 1874 ; Goette in Ai-chiv, 

 fiir Mikroskop. Anat., vol. x., 1874.) 



Partial seg-mentation in unfectindated ova. — It is proper further to state 

 that although the process of segmentation as now described is the necessary 

 preliminary to the formation of the blastoderm and is only complete in ova 

 which have been perfectly fecundated, yet an imperfect or partial kind of 

 segmentation has been found also to occur in unfecundated ova. This has now 

 been obsei-ved in a variety of animals, such as moUusca, fishes, batrachia. and 

 also in the mammiferous ovum (see Bischofi:, Ann. d. Sc. Nat. 1844. and Miiller's 

 Archiv. 1847 : Leuckart, article '• Zeugung " in Wagner"s Handwoiterbuch der 

 Physiol., 1852). Oellacher has recently investigated these phenomena with 

 care in the egg of the fowl, from which it appears certain that some degree of 

 segmentation of the germ does occur in unfecundated ova, but that it is of an 

 irregular and incomplete kind as compared with that which follows impregnation, 

 that it never goes on to the formation of a complete cellular blastoderm, and that 

 although some of the earlier stages of segmentation are gone through and the 

 g-erm is to some extent divided into segment areas, yet these are afterwards 

 ijroken up by vacuoles and other unnatural processes of development, and no 

 true blastodei-mic layer of cells is fonned (fig. 49.5, c). Enough, however, has 

 been seen to show that some formative power resides in the germinal part of 

 the yolk independently of the concurrence of the male element. It is not 

 improbable that this segmentation in unfecimdated ova may occur to a greater 

 extent in the lower than in the higher animals. 



2. THE BLASTODERM ; ITS STRUCTURE AND RELATION TO THE 

 DEVELOPMENT OF THE EMBRYO. 



Position and extent. — It has already been stated tliat in the bird's 

 egg the result of segmentation is the conversion of the germinal disc 

 into an organised cellular blastoderm, Avhich, from the time of its first 



