DAVENPORT : PROCESSES CONCERNED IN ONTOGENY. 



175 



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

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

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

 tion, — threads, fibres, tubules ; (III.) 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.^ 



I. Ontogenetic Processes occurring in Migratory Protoplasmic 

 Bodies — Mesenchyme.^ 



1. Migration of Nodal Thirkeninfjs in a Protoplasmic Mes/i-work. This 

 process is found, for exam})le, in 

 many Arthi'o|)od eggs before the for- 

 mation of the peripheral blastoderm. 

 (Figiu'e 1.) No one can doul>t that 

 protoplasm extends throughout the 

 whole egg in the form ot a mesh- 

 or foam-work, whose interspaces are 

 filled with yolk. Tiie 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.,8 Kigs. 7, 363, 417, 448, 472, 

 473, 771. 



n.t. 



Fig. 1. 



1 It may be a cnuse of (lissatisfaction to some tliat this elassifioation is not 

 "strictly dicliotoiiioiis " ; it is siill mors serious tiiat tiie ditTereiit heads are not of 

 co-ordinate rank or mutually exclusive. Of course, the classification emph)yed in 

 this list cannot be regarded as a final one. I ho[)e, however, that I liave 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 amro- 

 boid, migrating cells, of whatever origin. 



8 'l''hr()ugh()ut this paper certain abbreviations arc used in referring to the books 

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

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

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



Fig. 1. Section through an v{;<x of a Myriapod (Oeopliilns), sliowing the nodal 

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

 K. & II., Fig. U<). 



