i SEGMENTATION 7 



tending to lag, in their mitotic division, more and more behind the 

 less yolky upper elements. This inequality is found at its maximum 

 in the large eggs of Elasmobranchs, Eeptiles, and Birds, where the 

 main mass of the egg has its proportion of protoplasm reduced so 

 nearly to vanishing point that it does not divide at all. It is 

 only a small portion of the egg in the neighbourhood of the apical 

 pole that is rich enough in protoplasm to carry out the process 

 of segmentation into separate cells. This is known as the germinal 

 disc or, later on, when it has segmented into a mass of cells, 

 blastoderm. An egg of such a type, showing partial or incomplete 

 segmentation, is termed meroblastic in contrast with the more 

 primitive holoblastic type in which the egg segments as a whole. 



The blastomeres into which the egg divides being composed of 

 protoplasm — a somewhat viscous fluid- — tend under the physical 

 laws of surface tension to assume a spherical shape except when 

 flattened by pressure against their neighbours. There thus exist 

 normally chinks between the blastomeres filled with watery fluid. 



As the process of segmentation proceeds this intercellular fluid 

 increases in amount and the process normally culminates in the 

 stage known as the blastula. The blastula consists of a more or 

 less spherical mass of cells surrounding a relatively considerable 

 volume of fluid which is for the most part no longer distributed in 

 small chinks but collected together into a large space — the blastocoele 

 or segmentation cavity. 



In the simplest case, that of Amphioxus, the wall of the blastula 

 is composed of a single layer of cells — the cells towards one pole 

 being larger and containing fine granules of yolk or food material. 

 In holoblastic Vertebrates above Fishes it is however, as a rule, no 

 longer composed of a single layer, the roof of the segmentation 

 cavity being frequently composed of two layers while the floor is 

 composed of a thick mass of large heavily yolk-laden cells. 



The details of the segmentation process may now be followed 

 out as it occurs in the various types of lower Vertebrates. 



AMPHIOXUS 



Amphioxus is, of all the lower Vertebrates, that in which 

 developmental processes are least interfered with by the presence 

 of yolk, and for this reason the phenomena shown during its seg- 

 mentation must form the basis for the comparative study of the 

 corresponding phenomena in the Vertebrata in general. 



The process of segmentation in Amphioxus was described first 

 in two works which are now amongst the classics of morphological 

 science: the first by A. Kowalevsky (1867) and the second by 

 B. Hatschek (1881). 



The process begins (Fig. 3) with the appearance of a depression 

 of the surface in the region of the apical pole. This depression 

 takes an elongated groove-like form and extends outwards at each 



