i SEGMENTATION 25 



after fertilization, with the successive appearance, at the apical pole, 

 of two meridional furrows (a and /3) which gradually sweep down- 

 wards to the opposite pole of the egg. Before they reach it, four 

 vertical furrows make their appearance, commencing at a point on 

 furrow a not far from the pole and gradually extending downwards 

 over the lower part of the egg (Fig. 14, B 1). 



Before these vertical furrows reach the lower pole a new furrow 

 — latitudinal — develops a short distance from the apical pole, 

 marking off a polar group of eight micromeres (Fig. 14, B 2). At the 

 next division these divide into a superficial and a deep segment (the 

 former being separate — the latter continuous with the yolky mass 

 beneath) while the macromeres divide by vertical furrows. 



Next an irregular latitudinal furrow develops below the previously 

 existing one, by which a new micromere is segmented off from the 

 upper end of each macromere. 



Lepidosteus.— The ellipsoidal or " oval " egg measures about 3 -5 mm. 

 by 3-2 mm. and has a cap of protoplasm with fine grained yolk at its 

 apical pole. 



Segmentation (Fig. 14, C) in its early stage is like that of Amia 

 except that the furrows are more sluggish in spreading downwards 

 over the egg-surface. They never in fact reach much beyond the 

 equator; in other words, in the case of Lepidosteus, the lower 

 hemisphere of the egg does not normally segment at all." The egg 

 therefore has advanced beyond' the condition seen in Amia and has 

 become meroblastic. 



In the later stages of segmentation the region of the upper pole 

 is occupied by a lenticular mass of blastomeres which may be termed 

 the blastoderm, and this is bounded at its lower edge and over its 

 lower surface by a set of elements which remain in continuity with 

 the yolk. Later on the divisions between these elements tend to 

 disappear and their place becomes occupied by a " yolk-syncytium " 

 containing numerous nuclei. 



To summarize then, we have exemplified by the three ganoids 

 Acipenser, Amia and Lepidosteus, three steps in evolutionary 

 change, associated with an increasing degree of telolecithality, from 

 the holoblastic type of egg met with in Lampreys or Amphibians or 

 Crossopterygians to the meroblastic type as it exists in modern 

 Teleosts. 



Lung-fishes. — The early stages of segmentation have been 

 observed in two out of the three still existing lung-fishes — Geratodus 

 (Semon, 1893) and Lepidosiren (Graham Kerr, 1900). 



In the case of Ceratodus the egg measures about 3 mm. in 

 diameter and is pigmented in the neighbourhood of the apical pole. 

 The first two furrows (Fig. 15, A 2 and 3) are meridional and at 

 right angles to one another. Each appears first at the apical pole 

 and extends downwards with varying rapidity. The third set of 

 furrows are vertical and make their appearance usually before the 

 second meridional furrow (/?) has reached the lower "pole. The egg 



