DAVENPORT : PROCESSES CONCERNED IN ONTOGENY. 



183 



stem strikes a cai'tilage or a vessel, its fibres are bent in difterent direc- 

 tions 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). 



Fig. 12. 



h. The second case, that of division of a tubule throughout its 

 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, 

 for instance, at the close of the process of growth 

 of the neck of a raesenchymatous gland (or other independently arising 

 tubule) towards its insertion. 



Fig. 13. 



Figf. 12. Sections at difTerent levels through the cardial aorta of a human 

 embryo of 11.5 mm. The lowest section is at the left: the hip;hest at the risht : 

 the separation progresses from above downwards, a, aorta; p, pulmonary artery. 

 See M., Fig. 20.3. 



Fig. 13. The cervical and three of the thoracic spinal nerves of a human embryo, 

 showing the origin of tiie thoracic plexus. See M., Fig. .300. 



I 



