GASTRULATION AND EARLIEST DEVE LOPMENT 181 



between the two is, when measured dorsally, perhaps 

 somewhat shorter than when measured ventrally, but the 

 difference is very insignificant and we may safely say that the 

 length of the dorsal embryonic rudiment is 180°. Although 

 in yolk-laden eggs this arch has a somewhat smaller length, 

 still the above relation occurs regularly in various animal 

 groups. Besides in Amphibia, we find it e g. in Teleostei; 

 especially in those with pelagic eggs not too yolk-laden, 

 it is generally observed that the closure of the blastopore 

 takes place almost diametrically opposite the animal pole, 

 i. e. the point of the nose, so that here also the embryo 

 extends over almost 180° between the animal and vegetative 

 poles. Also in the anchovy, as we have seen above, this is 

 the case. Far from a fundamental difference, as MORGAN 

 (1894) thought, we find a fundamental agreement in the 

 position of the embryo in Amphibian and Teleostean eggs. Also 

 for Amphioxus the same holds good. CerfONTAINE's (1906) 

 pictures of gastrulas of Amphioxus with the polar body 

 still attached to them show that here also the blastopore 

 ' after having contracted lies approximately diametrically 

 opposite the animal pole, while the dorsal blastopore lip is 

 as well formed here near the egg equator. 



We clearly see from fig. 356 that the place of the first 

 appearance of the blastoporic rim lies in about the middle 

 of the length of the embryo, that consequently the embryo 

 is formed half on the black, half on the white surface of 

 the egg after the latter has been overgrown by the dorsal 

 blastopore lip, and that the main axis of the egg coincides 

 with the longitudinal axis of the embryo, so that the second 

 cleavage of the egg in so-called typical development 

 (ROUX ^) separates the dorsal and ventral halves. Since in 

 the 4- or 8-celled stage the distance from the animal to 

 the vegetative pole (upper and lower crossing point of the 



^) In the fertilised but still unsegmented frog egg a bilateral 

 symmetry is soon to be observed, caused by the white — in the 

 form of the so-called "grey field" (Roux) — extending on one side, 

 the dorsal one, higher up towards the egg equator than on the 

 other side, in tlie majority of cases the first cleavage plane coincides 

 with the plane of symmetry of the unsegmented egg and the symmetry- 

 plane of the embryo is then in its turn the same as these two. 

 This way of development was called "typical" by Roux. It has been 

 shown (Brachet, 1905 a, Delsman, 1916) that in such eggs where 

 the first cleavage plane does not coincide with the symmetry plane of 

 the egg, the symmetry-plane of the embryo coincides with that of 

 the unsegmented egg, not with the first cleavage-plane. 



