DAVENPORT: PROCESSES CONCERNED IN ONTOGENY. 183 
stem strikes a cartilage or a vessel,.its fibres are bent in different direo- 
tiohs and the stem divides. The resisting objects are thus considered 
by His to act in a mechanical, i. e. direct way. The results are, however, 
equally explicable by the response-to-stimulus theory. 
The splitting which occurs at the blind ends of developing blood-ves- 
sels and excretory tubules, and the repeated divisions of many glands — 
salivary gland (M. Fig. 334), liver, and lungs (M., Fig. 445) — are exam- 
ples of this process. Other cases are found among Invertebrates, as, for 
example, the tentacles of many Cnidaria (K. & H., Figs. 27, 31), “roots " 
of Sacculina, and liver branches of Limulus (K. & H., Fig. 338). 
is 
ts 
ao 
b. The second case, that of division of a tubule throughout 
entire length, is illustrated in the development 
of the segmental duct in some Vertebrates, and 
in the separation of aorta and pulmonary artery. 
(Figure 12). 
3. Anastomosing, or the process of fusion of 
similar threads, thus forming a network, is of 
wide-spread occurrence. It is exemplified in the 
development of nerves (Figure 13) and blood-ves- 
sels in Vertebrates, and in some glands, especially 
the vertebrate liver. 
4. Fusion with other Organs. The process of 
fusion of diverse organs will be studied in greater 
/ 
detail in other sections of this paper. It occurs, 
Fie. 18. 
for instance, at the close of the process of growth 
of the neck of a mesenchymatous gland (or other independently arising 
tubule) towards its insertion. 
Fig. 12. Sections at different levels through the cardial aorta of a human 
embryo of 11.5 mm. The lowest section is at the left; the highest at the right: 
the separation progresses from above downwards. a, aorta; p, pulmonary artery. 
See M., Fig. 203. 
Y'g.13. The cervical and three of the thoracic spinal nerves of a human embryo, 
showin the origin of the thoracic plexus. See M., Fig. 360. 
