II MORPHOCHORETIC PATTERN — -FERTILIZATION 325 



hypothesis has received experimental support, and also explains the puzzling 

 exceptions to the v. Baer's rule (Glavert and Vintemberger, 1954, 1957, 1958; 

 Vintemberger and Glavert, 1958a, b). A similar line of investigation has been 

 started (Fig. 16) on reptile eggs (Pasteels, 1955b, Glavert and Zahnd, 1955). The 

 results although still limited, are in agreement with the preceding views. It may 

 seem astonishing that such processes of reorganization take place in these amniotes, 

 in eggs which are not only fertilized, but already divided into several blastomeres. 

 This is indeed a difficulty, but we must remember that in these telolecithic eggs 

 there exists, during the first steps of cleavage, a zone of cytoplasmic continuity 

 under the blastomeres, a device which might explain the general effect of the 

 orientated rotation. 



In mammals, as stated above, symmetrization can be demonstrated in several 

 species before fertilization. In spite of thorough studies (De Geeter, 1954; 

 Dalcq, 1955, 1956a) no modification of the pattern has so far been detected at 

 fertilization; however, the method used was not experimental, and perhaps not 

 completely adequate. It has been claimed (Ancel and Reiss-Brion, 1956) that in 

 the mouse conditions existing in the oviduct and uterus could influence the 

 orientation of the embryo. The problem is certainly not yet solved. 



If a conclusion may be drawn from this too limited inquiry, it seems probable 

 that the acquisition of a polarized and symmetrized pattern is generally realized 

 during oogenesis, although terminal growth may more or less veil these features. 

 In some cases, the site of fertilization may modify the previous organization and 

 impose a new plane of symmetry. In the same cases, subsequent manipulations 

 may again re-transpose the dorso-ventral axis. This mechanism acts normally in 

 birds and probably also in reptiles in order to impose the final symmetry of the 

 embryo. An analysis of these processes discloses a reciprocal influence between the 

 cortex and the endoplasm. This is one of the important consequences of fertilization 

 (seep. 343). 



C. Cleavage and morphochoretic pattern 



It has long been recognized that to a variable degree and according to groups 

 cleavage can be affected by the morphogenetic organization of the fertilized egg. 

 Among chordates, this correlation is most evident in tunicates; it was later found 

 to exist also in the Amphioxus (Gonklin, 1932) so that it is considered a charac- 

 teristic of protochordates. In amphibians, the relations are much looser, but in 

 cases where the first division is in the frontal plane, the dorsal blastomere is 

 usually smaller, which means that the first spindle was located more dorsally, 

 due to some influence of the light (or grey) crescent. If the first furrow is sagittal, 

 the second will also be displaced dorsalwards. In reptiles, and also in monotremes 

 (Flynn and Hill, 1939), cleavage may also show some symmetry, but the corre- 

 lations are not so clear. In the rat', the orientation of the first and second furrow 



^ cf. Dalcq, 1954b, 1955. A cytochemical reaction has now (1959) detected a cortical 

 differentiation with a field pattern, the maximum intensity corresponding to the dorsal 

 (embryonic) half of the egg. 



Literature p. 483 



