DAVENPOKT : PROCESSES COXCEhXED IN OXTOGEXY. 



175 



the protoplasmic bodies in which tbey occur. These exist either as 

 (I.) isolated cells, or as larger multinucleated bodies. Of the latter we 

 may recognize three classes : (II.) bodies extended chiefly in one direc- 

 tion, — threads, fibres, tubules ; (111.) those which extend as a layer; 

 and (IV.) those in which the three dimensions are more nearly equal, 

 forming solid masses. 



I propose now to discuss the processes occurring in each of these four 

 classes. 1 



Ontogenetic Processes occurring in Migratory Protoplasmic 

 Bodies — Mesenchyme. 2 



1. Migration of Nodal Thickenings in a Protoplasmic Mesh-work. 

 process is found, for example, in 

 many Arthropod eggs before the for- 

 mation of the peripheral blastoderm. 

 (Figure 1.) No one can doubt that 

 protoplasm extends throughout the 

 whole egg in the form ot a mesh- 

 or foam-work, whose interspaces are 

 filled with yolk. The protoplasm 

 is aggregated around the nuclei at 

 certain nodal points, which later 

 migrate to the surface or through 

 the yolk as vitellophags. Cf. K. 

 & H., 3 Figs. 7, 363, 417, 448, 472, 

 473, 771. 



This 



Fig. 1. 



1 It may be a cause of dissatisfaction to some that this classification is not 

 "strictly dichotoraous " ; it is still more serious that the different heads are not of 

 co-ordinate rank or mutually exclusive. Of course, the classification employed in 

 this list cannot be regarded as a final one. I hope, however, that I have succeeded 

 in an attempt roughly to arrange the different items in a logical fashion. 



2 In the present paper the word " mesenchyme " is used as a name for all amoe- 

 boid, migrating cells, of whatever origin. 



3 Throughout tins paper certain abbreviations are used in referring to the books 

 from which the figures are copied. These are : K. & H. for Korsehelt und Ileider's 

 " Entwicklungsgeschichte "; M. for Minot's "Human Embryology "; and II. -M. for 

 Hertwig's " Text-Book of Embryology of Man and Mammals," translated by Mark. 



Fig. 1. Section through an egg of a Myriapod (Ge'ophilus), showing the nodal 

 thickenings (n. t.) in the act of migrating towards the periphery of the egg. See 

 K. & II., Fig. 449. 



