'•>■) 



PROCEEDINGS OF THE ACADEMY OF [Oct., 



by Wilson (1892) for Aricia. In both cases this arrangement is prob- 

 ably brought about by the mechanical processes involved in spiral 

 cleavage. Each cell approximates in shape a four-sided prism, the 

 nuclei lying in the lower ends of the cells, near their ventral surface. 

 As the blastopore narrows, however, these nuclei begin to move 

 inward (figs. 54 and 56). This movement is associated with a change 

 in shape of the entomeres, which in turn is part of the process of gas- 

 trulation. As the blastopore lips draw together (figs. 53 and 55) the 

 lower or vegetal pole ends of the entomeres grow smaller, while the 

 upper or animal pole ends become larger (figs. 54 and 56). These cells 

 thus change from a prismatic to a pyramidal shape. Viewed in a sagit- 

 tal optical section their outlines radiate fanwise from the blastopore 

 (fig. 56). This condition is figured by Repiachoff (1886) and also by 

 Schimkewitsch (1895). This peculiar phenomenon may be a remi- 

 niscence of a time when the type of gastrulation was embolic and not 

 epibolic, and the latter condition may have been brought about by 

 acquisition of food yolk during the phylogeny. Such acquisition of 

 yolk is a secondary character commonly associated with a change 

 from larval to direct development, and it is perfectly possible that a 

 change from emboly to epiboly may have been brought about in this 

 manner. The nuclei have meanwhile undergone changes in structure 

 as well as in position. The chromatin, which before was distributed 

 in the nuclear vesicles in the form of small granules, is now concentrated 

 in each vesicle in one deep staining chromatin nucleolus (fig. 56). This 

 condition is not an uncommon one in resting cells, and is also seen in 

 the entomeres of Crepidula (Conklin, 1897, figs. 52, 53 and 54). 



The further history of the entoderm I have not been able to follow 

 in detail. At a period when the stomodseum has assumed the position 

 of the definitive mouth, the entoderm cells are seen to have multiplied 

 somewhat and to have assumed an arrangement quite different from 

 that found in earlier stages. In fig. 57, which is a horizontal, optical 

 section of a stage when the stomodseum is just making its appearance, 

 the entoderm cells have still the radial arrangement which they as- 

 sumed at the time of the closure of the blastopore. In fig. 59, a sagittal 

 section of a much later stage, the arrangement is totally different; 

 the entoderm cells have multiplied and are now arranged in a more 

 or less definite layer about a small central lumen. 



(2) The Mcsomeres. 



At the 29-cell stage 4d is but just formed and lies on the lower side 

 of the cleaving ovum, below and in front of X and to the left of the 



