966 FORMATIONS FROM THE EPIBLAST. 



Thus, there are developed in all vertebrates three germinal layers. 

 From the ectoblast there results the central nervous system, the epider- 

 mal formations, and also the epithelium of the organs of special sense. 

 From the hypoblast is formed the epithelium of the intestinal tract, 

 including the cells of the glands that result through evagination from 

 the intestinal tube. All of the other tissues of the body, with the excep- 

 tion of the parts forming the vascular system and the connective- 

 tissue substances, develop from the mesoblast. 



The cells of the ectoblast, but particularly those of the hypoblast, take up 

 during development, in the bird, the constituents of the yolk through direct active 

 incorporation, and in this process the ameboid movement of the cells plays a role. 

 The parts taken UJD are transformed (digested) within the cells, and employed 

 in the process of building up. 



The division of the cells of the growing tissues takes place in the following manner: 

 i. By direct cell-division, in which first the nucleus and then the cell-body breaks 

 up into two halves, for example in the division of the embryonal erythrocytes 

 (page 41); 2, by indirect cell-division (mitotic division), in which the following 

 processes are observed in the cell: (a) The nucleus becomes enlarged and its 

 chromatin-network increases and takes on a definite grouping. There form 

 loops, which at one pole of the nucleus (polar field) exhibit especially bendings, 

 and at the other (opposite polar field) the extremities of the limbs of the loops. 

 This is the stage of the close skein or the spireme. (6) The close skein is trans- 

 formed into the looser, the threads becoming separated. In this process part of 

 the loops turn toward one pole, and part to the other (segmented skein, c). 

 (c) All the loops move with their bendings toward the center; and there is thus 

 formed the star (first mother-star, d). (d) Meanwhile, there is formed the achro- 

 matic nuclear spindle, which bears at each extremity a polar body, from which the 

 nuclear protoplasm passes in a radiate manner polar rays (d, e, /) . (e} The loops 

 divide lengthwise; each half moves away from its fellow (e, /). (/) The loops 

 undergo a rearrangement metakinesis (e, /), and form two equatorial plates. 

 (g) After atrophy of the connecting threads, the protoplasm of the nucleus, and 

 later also that of the cells, undergoes division, and the network of both nuclear 

 halves (dispireme, g) appears as in the original form of the undivided nucleus (a) . 



The cell consists of body, nucleus, and nucleolus. The cell-body forms a mov- 

 able protoplasm, which appears as a threadwork, or network, or honey-combwork 

 in the midst of which, in a softer substance, lie small granules. The nucleus 

 possesses a capsule, and consists of a nuclear ground-substance, in which (color- 

 able) chromatin and achromatin lie as enclosures. The nucleolus is a dense 

 mass of chromatin ; Occasionally, accessory nucleoli are present (Flemming's 

 reticular nodes). Finally, the cell-body contains also the centrosome, surrounded 

 by an area, the actual center of motion of the cell, which also breaks up in the 

 process of cell- division. All cells are derived from parent-cells; Omnis cellula 

 ex cellula (Virchow). From cells at first apparently similar the elements 

 of the different organs and tissues develop through transformation. If, therefore, 

 all tissues are referable morphologically to a single form of primitive cell, it 

 follows that the physiological activity of the different organs and tissues must 

 be referred to a single primitive form of function, to an "identity of physiological 

 activity, ' ' present from the beginning. The proof of the development of the special 

 activity of the tissues from this as yet un differentiated primitive form of vital 

 manifestation will, with certainty, at a later period constitute an important chapter 

 of the subject of physiological development. 



FORMATIONS FROM THE EPIBLAST. 



Upon the ectoblast there is formed, in mammals, as in birds, in front 

 of the primitive streak, and at a later period, a longitudinal furrow 

 ( Fl g- .373, IV, and Fig. 374, D), whose margins, curved anteriorly, pass 

 over into each other ; while posteriorly they pass side by side, though in 

 a somewhat divergent manner. This is the medullary or spinal groove. 

 Later on the adjacent margins, the medullary or spinal folds approach 

 each other at their free edges, and finally join in the median line, to 



