192 Proceedings of the Royal Thy deal Society. 



Gastrulation. 



The process of gastrulation in the forms under considera- 

 tion is of interest, as affording links between the types of 

 gastrulation familiar in other Anamnia. In Polypterus the 

 process, so far as is possible to judge from the meagre 

 material available, shows characters which, while resembling 

 those found in Amphibians, have not advanced quite so far 

 along the path of specialisation from the primitive mode 

 of gastrulation found at the present time in Amphioxus 

 alone amongst vertebrates. In Lejndosirm, on the other 

 hand, the mode of gastrulation, while again resembling the 

 process as met with in the Amphibia, shows a decided 

 advance towards the type of gastrulation found in the 

 Elasmobranch. 



Polypterus. 



The earliest stage known in the process of invagination is 

 one where a sharply-marked groove extends about half-way 

 round the equator of the egg. In a later stage, the groove 

 forms a complete circle about 50 degrees from the lower 

 pole. The diminution in radius of curvature, and the still 

 circular form of the curve, probably indicate that the line 

 of invagination has been advancing downwards towards the 

 lower pole at an equal rate at all parts of its extent. The 

 specially characteristic feature at this stage is the enormous 

 unpigmented yolk-plug, sharply marked off by the deep 

 and wide groove from the remaining pigmented part of the 

 egg surface. In this stage we see a transitional condition 

 between that found in Amphioxus, where the line of invagi- 

 nation forms a complete closed curve while still nearly 

 equatorial in position, and that characteristic of the 

 Amphibian, where it is completed only at a much later 

 stage, when it has advanced much farther over the egg 

 surface, and had its radius of curvature greatly reduced. 

 In these modifications of the gastrulation process we prob- 

 ably have to do with a postponement of the process caused 

 directly by the increased quantity of yolk. In AmphioxuSy 



