DEVELOPMENT OF LYMPHATICS IN ANURA 79 
into trabeculae, which become more and more attenuated, and 
finally break and disappear as the sinus becomes more greatly 
distended in its vertical, that is, dorsoventral, diameter. ‘These 
successive steps are clearly exhibited in the inserted photomi- 
crographs, figures 11, 12, and 13, the last two illustrating how 
the mesenchymal strands are drawn out and tear and how their 
remnants persist for a time as longer or shorter spurs which pro- 
ject into the sinus cavity. 
During the further growth and enlargement of the sinus, I 
was unable to find the addition of separate mesenchymal spaces 
by concrescence, such as I described in the development of the 
thoracic duct in the pig (’12) or those of McClure (15) in the 
formation of the subocular lymph sac in the trout, or those of 
Huntington (11) on the growth of the periaortic lymphatics in 
Chelydra. 
I ean but believe that the coalescence of the originally dis- 
continuous lymphatie anlagen, the formation of the intricate 
lymphatic plexus and its conversion into the relatively enormous 
sinus is largely, perhaps wholly, due to the accumulation within 
their lumen of lymph, which, as it increases in quantity, increases 
the internal pressure on its walls and achieves the extension and 
distention of the developing sinus, for during this important genetic 
period the sinus possesses no outlet; it is not confluent with the veins. 
The saccular and expanded posterior prolongations of the tem- 
poral plexus shown in the reconstruction (fig. 30) certainly point 
to such an interpretation. A similar view was expressed by 
McClure (’15) in his preliminary paper on the development of 
the anterior lymphatics in teleost embryos. 
Coincident with the expansion of the lymph sinus, its lining 
cells assume all of the attributes of typical endothelia. The cells 
become much flattened, and their nuclei, which in earlier stages 
resembled those of mesenchyme in their spherical shape and their 
coarse chromatic texture, become more and more compact and 
dense like the intimal nuclei of older vascular channels. The 
yolk corpuscles in the cytoplasm of the endothelium also gradu- 
ally disappear, although in 9- and 10-mm. embryos a few are 
still to be found. 
THE AMBRICAN JOURNAL OF ANATOMY, VOL. 30, No. 1 
