Section VI 



CHAPTER 2 



Teleosts and Birds 



DOROTHEA RUDNICK 



In the course of divergence from primitive 

 vertebrate stock, two modern groups — the 

 fish and the sauropsidans — have separately 

 become characterized by a large-yolked egg 

 as a framework for ontogeny. Necessarily the 

 eggs of both groups share certain mechani- 

 cal adaptations: the discoid meroblastic 

 cleavage pattern, the ultimate envelopment 

 of the yolk by the spreading protoplasmic 

 disc. These likenesses are of such superficial 

 nature as scarcely to be likenesses at all, 

 morphogenetically speaking. Movements of 

 material — that is to say, the relative situa- 

 tion of prospective areas of the disc — differ 

 radically in the two classes of egg. The 

 characteristic vertebrate axis that eventuates 

 is in each case achieved by a quite different 

 morphogenetic sequence. The adnexa, by 

 the same token, differ strikingly in their 

 origin and relations. 



In contrast, the experimental analysis of 

 independent behavior of various parts of 

 these blastoderms has tended to emphasize 

 the basic likeness of development in all 

 vertebrate groups. Where comparable ex- 

 periments have been performed on eggs of 

 fish and bird, the results seem likewise to 

 be comparable. There is so far no suggestion 

 of divergence in the fundamental pattern of 

 differentiation processes. 



THE PRE-CLEAVAGE PERIOD 



The massive telolecithal eggs, like other 

 eggs, are started in the main course of their 

 genesis in the peculiarly specialized en- 

 vironment of the ovary, in relation to other 

 cells and to maternal body fluids. Significant 

 experimental interventions in this complex 

 situation are extremely difficult to realize. 

 From cytological and cytochemical studies 

 it seems clear that not only a surface-interior 

 differential, but morphological polarity ap- 

 pears fairly early in the growth period; 

 it may be assumed that, like other animal 

 eggs, these leave the ovary with at least 



provisional axes of polarity and symme- 

 try established. The effect of the sperm 

 at fertilization on this pattern is not 

 known. 



In the teleost egg, the early radial pattern 

 of the primary oocyte is reinforced by a 

 gradual migration of material from the 

 periphery of the nucleus to the cell mem- 

 brane where a cortical layer is differentiated. 

 This migration has been observed by Spek 

 ('33) and inferred by Brachet ('44, p. 72) 

 from figures in the literature; whether more 

 than one material or process is involved in 

 these diverse observations is not clear. Yolk 

 formation follows this radiating phase, ac- 

 companied by marked increase in the diam- 

 eter of the cell. Sometime during this period 

 — evidently not at first — eccentricity of the 

 nucleus is observed; this body eventually 

 comes to rest against one pole of the egg 

 cortex, the micropyle region, and maturation 

 is initiated (Spek). The interpretation sug- 

 gested by these accounts is that the basic 

 cortex-interior pattern, reinforced during the 

 early radial period when the nucleus is 

 visibly active with reference to the cyto- 

 plasm, is given a slight axial differential 

 at this time, and that the oocyte grows on 

 this framework as a polarized system. Sub- 

 sequently, as in many eggs, after release into 

 water, active streaming of cytoplasmic ma- 

 terial establishes a sharp layering along the 

 polar axis: protoplasmic cap, basal vacuolar 

 area, yolk, and usually "oil" droplets. Many 

 teleost eggs display great surface activity 

 at this time and during cleavage (cf. Yama- 

 moto, '38, etc.; Roosen-Runge, '38; Lewis 

 and Roosen-Runge, '42); several workers 

 have interpreted results of injury and trans- 

 plantation experiments (see p. 299) as show- 

 ing that not only does material continue to 

 stream into the blastodisc during cleavage, 

 as is apparent in the Lewis-Roosen-Runge 

 film, but that this material is of critical 

 morphogenetic significance. However, no- 

 body as yet has observed the actual incor- 



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