DEVELOPMENT OF THE BLOOD VESSELS IN THE FROG. 205 



trunks and lymphatic spaces of Amphioxus, on which Lankester* has 

 recently laid stress. 



v. The afferent branchial vessels develope subsequently to the efferent 

 vessels. At 4^ mm. afferent vessels are present only in the first and 

 second branchial arches, lying immediately behind the corresponding 

 efferent vessels. The afferent vessel arises as an irregular lacunar 

 space opposite the base of the external gill; it is at first independent, 

 but soon acquires connection with the efferent vessel by narrow 

 lacunar passages in the substance of the gill process, which passages 

 give rise later on to the capillary loops of the gills. The ventral 

 part of the afferent vessel extends downwards behind the efferent 

 vessel, towards the truncus arteriosus, but does not at 4^ mm. reach 

 so far down as the efferent vessel. 



vi. At i\ mm. there are very few blood corpuscles present. The 

 heart in many specimens has none, but in others a few may be seen. 

 The dorsal aorta may or may not contain corpuscles ; but in the 

 efferent branchial vessels a few are always present. These corpuscles 

 are spherical nucleated cells, richly laden with yolk granules ; they 

 resemble very closely the cells of the surrounding tissues, and we 

 have obtained direct evidence that in the efferent branchial vessels 

 they are formed in situ, from the walls of the vessels themselves. 



The description given by Maurerf of the development of the 

 vessels in B,ana esculenta differs in some important respects from the 

 above account. 



According to Maurer, the efferent branchial vessels appear as 

 lacunar spaces in the connective tissue of their respective arches, 

 and have at first no connection with the heart ; while the dorsal 

 aorta is not yet formed. The efferent vessels soon acquire com- 

 munications with the truncus arteriosus, and a little later with the 

 dorsal aorta. There is therefore a stage in which there is a single 

 vessel in the arch, forming a direct communication between the heart 

 and the aorta. As the development of the vessels commences earliest 

 in the anterior arches, this stage is met with in succession in the 

 arches from the first branchial to the fourth branchial. It will be 



* Lankester, "Contributions to the knowledge of Amphioxus lanceolatus," 

 "Quart. Journ. of Micros. Science," xxix., 1889, p. 379. 



t Maurer, " Die Kiemen und ihre Gefasse bei Anurenund Urodelen Ampliibien," 

 "Morphologisches Jahrbuch," xiv., 1888, pp. 178-184. 



