i SEGMENTATION ■ 23 



there is no longer any budding off of cells, the nuclei simply lying 

 within the substance of the syncytium. ■■ 



As they increase in number nuclei from the central and marginal 

 regions spread into the intermediate zone, which up to now 

 contained very few nuclei, while others pass outwards into the 

 peripheral protoplasm (Periblast — Agassiz and Whitman, 1885) 

 lying outside the limits of the blastoderm. 



Towards the end of the second day the syncytial nuclei begin 

 to increase markedly in size and they begin to undergo abnormal 

 multipolar mitoses. During the third day they complete the 

 assumption of these peculiarities which are characteristic of the 

 nuclei of a yolk-syncytium — enormous size.curiously lobed appearance, 

 and the tendency for the lobes to become nipped off irregularly so 

 as to give rise to groups of small nuclei. 



During these later stages of segmentation the blastoderm becomes 

 flattened somewhat and instead of bulging out over, its attached base 

 all round, its surface passes into the extrablastodermic surface by 

 a slope very much as it did before segmentation began (Fig. 13, F). 



Actinopterygian Ganoids. — The ganoid fishes are of special 

 embryological importance because, so far as actinopterygians are 

 concerned, they appear to be the least modified descendants of those 

 ancestral forms from which the Teleostean fishes have been evolved. 

 Study of their developmental phenomena is desirable in order to 

 see to what extent they throw light upon the peculiarities of 

 development which characterize the Teleostean fishes. It will be 

 necessary therefore to review the segmentation processes so far as 

 they are known in each of the three types — the Sturgeon, Amia and 

 Zepidosteus. 



The only Sturgeons of which anything is known regarding their 

 early development are the common sturgeons of the genus Acipenser. 

 Polyodon, Psephurus and Scaphirhynchus are so far completely 

 unknown, though it is highly desirable that their development should 

 be investigated. 



In both Acipenser ruthenus (Salensky, 1878) and A. sturio 

 (Bashford Dean, 1895) the segmentation (Fig. 14, A) is of the same 

 general type. The unsegmented egg measures about 2 mm. in 

 diameter in the Sterlet {A. ruthenus), about 2-8 mm. in the Sturgeon 

 {A. sturio). The lower part of the egg contains coarse yolk granules 

 while in the region of the apical pole it is richer in protoplasm and 

 the yolk is more finely granular. The first furrow (a) is meridional, 

 appearing first at the apical pole and gradually spreading downwards 

 and at the same time cutting more deeply into the yolk. The second 

 furrow (f3) is similar and at right angles to the first. The third set 

 of furrows seem to be typically vertical (A 2) but they show 

 much variation and may be practically meridional or may show a 

 tendency to be latitudinal. The next set of furrows again vary between 

 vertical and latitudinal and from now onwards there is no apparent 

 regularity in the segmenting of the various blastomeres. There 



