. Text-book of embryology. Embryology. 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
. Text-book of embryology. Embryology. 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 ; 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 featuresisuggest that in the an
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