DAVENPORT: PROCESSES CONCERNED IN ONTOGENY. 



177 



of Turbellaria according to Iijima (Zeitschr. f. wiss. Zool., XL. 455). 



Among Vertebrates, we have the observations of Paterson, (Figure 3,) 

 according to which the sympathetic nerve arises by the aggregation of 

 mesenchymatous elements into a strand; of His, who affirms the origin 

 of the spinal and the olfactory ganglia from migrating cells; and of 

 various authors, who make blood capillaries and lymph vessels arise by 

 this process (cf. M., pp. 217, -±13). 



Fig. 4. 



Fig. 3. 



b. Next we must consider the aggregation of mesenchyme into a 

 superficially extended body, — the formation of a layer. This process 

 does not seem to be very common ; one example is seen in Figure 4. 



c. As the last of these processes of aggregation we have the case of 

 aggregation into a mass. This wide-spread ontogenetic process may be 

 illustrated by the formation of gemmules in a marine sponge (Figure 5). 

 Other examples are found in the formation of the adductor muscles of 



Fig. 3. Cross section of a rat embryo in the upper thoracic region, showing the 

 development of the sympathetic nerve (between spinal nerve and carotid artery). 

 From A. M. Paterson, '91, Trans. Roy. Soc. Lond., PI. XXII. Fig. 4. 



Fig. 4. Later stage of the embryo shown in Figure 1. The migrating proto- 

 plasm has aggregated itself into a layer at the surface of the embryo. See K. >.<. II., 

 Fig. 449. 



Fig. 5. Section of a marine sponge (Esperella), showing a gemmule, a mass of 

 aggregated mesenchyme which is about to produce a gemmule, (1), and migrating, 

 not yet aggregated mesenchyme (1). After II. V. Wilson ( ',•4, Jour, of Morph., IX. 

 PI. XIV I 



