i SEGMENTATION 29 



segments more slowly — during at least the first stages— than does 

 that of the Anura. It may be said also that, on the whole, eggs with 

 a large mass of yolk show a tendency for the first latitudinal furrows 

 to be nearer the apical pole, so that the micromeres which they cut 

 off are relatively smaller. Also it seems to be the case that in the 

 lower, more yolky, parts of the egg the latitudinal furrows are 

 retarded to a particularly great extent, so that in such heavily yolked 

 eggs there is frequently visible a preponderance of vertical and 

 meridional furrows in the lower parts of the egg. 



Elasmobranchii. — Of the more typically meroblastic vertebrates 

 the Elasmobranchs call for little in the way of further remarks. 

 The general features of their segmentation have already been 

 described (p. 12). 



The eggs of all Elasmobranchs hitherto investigated are of large 

 size and undergo a meroblastic segmentation. Up to the present 

 time no Elasmobranch has been discovered in which the eggs are 

 small and holoblastic, though it is quite possible that such forms 

 exist. It need hardly be said that if they do the study of their 

 embryology will be of extraordinary importance as it will be of the 

 greatest help in enabling us to disentangle those developmental 

 phenomena of Elasmobranchs which are primitive from those which 

 are merely secondary modifications due to the accumulation of yolk. 



Saxjropsida. — The Sauropsida, like the Elasmobranchs, possess 

 large and richly yolked eggs with a meroblastic segmentation, but 

 the process of segregation of yolk and protoplasm has not been 

 carried to such an extreme as in Elasmobranchs, not to mention 

 Teleosts. A germinal disc is present but this still contains a con- 

 siderable amount of yolk and at its periphery passes by much more 

 gradual transitions into the main mass of yolk. Further in the 

 more primitive Eeptiles the blastoderm frequently occupies a much 

 larger proportion of the whole egg than it does in the Elasmobranch. 



The general features of segmentation resemble those of Elasmo- 

 branchs though the earliest phases depart in many cases less than 

 they do in Elasmobranchs from what is seen in holoblastic eggs. 

 Thus the process may commence with the appearance of a meridional 

 furrow followed by a second at right angles to it and then by two 

 pairs of vertical furrows very much as in an actinopterygian ganoid 

 (Fig. 14, B and C). 



This is seen most clearly in the less specialized egg of Eeptiles. 

 Even in the Eeptile however the process is liable to become irregular 

 'at an early stage by the reduction of particular furrows or their 

 irregular orientation. In the Birds (Patterson, 1910) the irregularity 

 is still more marked and even the third set of furrows may no longer 

 be clearly recognizable. 



As in the case of other bulky and heavily yolked eggs poly- 

 spermy appears to occur normally and an abortive accessory segmen- 

 tation may make its appearance round the accessory sperm-nuclei. 

 As in the Elasmobranch (Fig. 8, B*) this is only a transient 



