52 AMPHIOXUS. 



the top and bottom tiers the blastocoel is no\v completely 

 closed. 



From this time the blastomeres continue to increase in 

 number by division, but in less regular fashion than before, so 

 that the arrangement in tiers soon becomes lost : the blastomeres 

 at the lower pole, however, remain thi'oughout of larger size 

 than those in other parts of the embryo. The polar body is 

 often visible, resting on the upper pole of the egg, but it has 

 sometimes disappeared by this stage. The blastomeres, which 

 have hitherto been of somewhat irregular shape, rounded at 

 their outer and inner ends, and flattened through mutual pres- 

 sure at their sides, now begin to assume more definite form ; 

 and from this stage, which marks the close of segmentation, 

 they may be more appropriately spoken of as cells. 



4. The Blastula. 



The embryo has now reached the stage to which the name 

 hlastula is given ; a stage which occurs at corresponding periods 

 in the development of a number of the lower animals, and which 

 is therefore of interest as possibly representing a very early 

 ancestral form of animal life. Pandorina and Volvox are exam- 

 ples of organisms in which the blastula stage forms the adult 

 condition. 



The blastula (Fig. 14, vui) is a spherical or ovoid embryo, 

 consisting of a single layer of cells, inclosing a central segmen- 

 tation cavity or blastoccel, filled with fluid. In the blastula of 

 Amphioxus the cells are not all of equal size, those of the lower 

 half, and especially those at the lower pole, being distinctly 

 larger than those of the upper half; the greater size and more 

 opaque appearance of these lower cells are due to the greater 

 quantity of yolk granules which they contain. At first, the cells 

 of the blastula, though flattened laterally where they press 

 against one another, remain rounded at their ends, both inner 

 and outer. These ends, however, soon become flattened ; and 

 the cells, in which the nuclei are now clearly visible, acquire 

 the characters of columnar epithelial cells. These changes 

 appear first at the upper pole of the embryo, and gradually 

 extend to the lower pole. The blastula stage (Fig. 14, vui) is 

 reached by the Amphioxus embryo at about the end of the 

 fourth hour from the time of fertilisation of the eggs. 



