380 



DR ARTHUR T. MASTERMAN ON THE 



The production of the furrows and the appearance of the tracts can only be due to a 

 movement, either inwards or outwards. At first I was inclined to believe that it is a 

 case of formation of the diploblastic stage by multiple ingression, but my opinion 

 now is that it is an interesting process which we may term egression, and that the cells 

 (or nuclei) are really migrating outwards to the periphery. The reasons for this 

 conclusion are these : — The process is immediately succeeded by complete and total 

 gastrulation in which the outer layer is well-formed, whilst the blastocoele space 

 and the archenteric cavity are devoid of cells and nuclei (fig. 18) : thus, if it is 

 a process of multiple ingression, why is it followed by true gastrulation ? Again, it is 

 noticeable that as the migration proceeds, the outer layer becomes more and more 

 defined, the number of nuclei increases, and the cell-walls become better developed, 



D 



1, — Diagram of the Process of Egression. — (A) Section through a simple 

 morula, inner cells shaded. (B) Section through the egression stage. 



(C) Section through the blastula, with the former inner cells shaded. 



(D) Section through a more complex morula, e.g., Cribrella. (E) The 

 egression stage. (F) The fully formed blastula ; former inner cells are 

 shaded. 



whereas if it were a case of ingression the reverse should hold. Similarly, the progress 

 of the migration reduces the number of the internal nuclei. 



For these reasons I would hold that a peculiar process of cellular egression is 

 instituted with the evident purpose of producing a blastula from the morula. 



In the simple morula of a few cells, such as occurs commonly in development, the 

 single internal cells must be assumed, during the course of segmentation, to wander out- 

 wards and take up their place between the outer cells, producing a typical blastula. 

 In such a complex morula as that of Cribrella the large quantity of internal cells ne- 

 cessitate the cellular egression being conducted in a more complex manner, involving the 

 formation of cell-aggregates, which roll outwards and thus bring their constituent cells to 

 the surface. The accompanying figure will illustrate the process. 



We may assume that a morula stage is a ccenogenetic derivative of the blastula 

 produced by conditions of economy of space, in obedience to the same laws of growth as 



