SEGMENTATION 



19 



have been described by Bashford Dean (1899) and as might be 

 expected the segmentation is meroblastic. Apparently the first 

 two furrows (a and /3) have the normal meridional arrangement 

 the specimen figured by Dean (Eig. 10, A) showing a displacement 

 at the intersection of the two furrows. These latter do not reach 

 the edge of the germinal disc. The third set of furrows (Fig. 10, B) 

 appear to be vertical and in the next stage figured (Fig. 10, . C) 

 the. furrows have become joined up to form an irregular network 

 which still barely reaches the edge of the blastoderm. 



Crossopteeygians. — Our knowledge rests entirely on the 

 observations of Budgett (Graham Kerr, 1907). These, necessarily 

 fragmentary, observations suffice to show that the process of 

 segmentation is of great interest. In the earliest stage observed, 

 but not figured, by Budgett the egg was " segmenting in four equal 



Fig. 11. — Segmentation and gastrulation in Polypterus. 

 (drawing's by Budgett. Graham Kerr, 1907.) 



A, represents a view of the apical pole : the remainiDg figures are side views. 



portions, the constrictions being deeper than in the frog." A 

 second egg (Fig. 11, A and B) is in the eight-blastomere 'stage. The 

 blastomeres are practically equal in size and it may be inferred with 

 considerable probability that in Polypterus two meridional furrows 

 are succeeded by a latitudinal one which is very nearly equatorial. 

 The nearness of the latitudinal furrow to the equator is remarkable 

 in view of the fact that the egg of Polypterus, as shown by the 

 study of sections (Fig. 1, B, p. 3), is not by any means nearly 

 isolecithal. 



Actinopterygians. — The typical Teleost is characterized by the 

 fact that its richly yolked eggs show a more complete segregation 

 of protoplasm and yolk than do those of any other Vertebrata. In 

 correlation with this the segmentation is here the most markedly 

 meroblastic in character. These featuresi suggest that in the ancestral 

 Teleost the yolk was large in quantity and that the egg as a whole 

 was of great size. Amongst present-day Teleosts however it is only, 

 comparatively speaking, a few forms mostly inhabiting fresh water, 



