172 OTTO FREDERIC KAMPMEIER 



confluent. Others continue to grow medially until they meet 

 those coming from the opposite side. In this manner a very 

 complicated plexus of interanastomosing lymph vessels is estab- 

 lished in the ventral territory of the head. This plexus, as 

 reconstructions of it show, is remarkable for its regularity and 

 symmetry, roughly suggesting the form and extent of the finished 

 lymph resevoir. Such a phase has been reached in the younger 

 8 mm. embryos. The channels constantly distend and in effect 

 the walls of adjacent ones are rapidly approximated so that the 

 mesenchyme filling the meshes of the lymphatic network is 

 diminished in amount. In older 8 mm. embryos the sides of 

 contiguous vessels have met and begin to break down and dis- 

 appear at the place of coincidence; in this way the plexiform 

 character of the developing sinus is progressively obliterated. 

 Transverse sections at this stage show many mesenchymal and 

 endothelial strands and partitions crossing the cavity of the 

 sinus dorso-ventrally and giving to it a multi-locular appearance, 

 which compares favorably with a similar transient phase in the 

 embryonic history of the thoracic duct in the pig. Such trabec- 

 ulae are still conspicuous in 9 mm. specimens, but later these 

 last vestiges of boundaries between originally independent chan- 

 nels vanish and leave the sinus a vast uninterrupted lymph 

 chamber. 



During these genetic processes, the yolk content of the lym- 

 phatic anlagen has suffered marked diminution, and coexistent 

 with this change the intimal cell has passed through a gradual 

 metamorphosis from the stage of an undifferentiated generalized 

 cell to that of a specialized endothelial-like cell. In 10 mm. 

 larvae a casual yolk spherule may still be found in the confines 

 of the sinus, and the lining* cells have assumed the features typical 

 of all well-formed endothelia. From now on the prime alter- 

 ation which the lymph sinus undergoes is the establishment of 

 cont nuity between it and the other components of the lymphatic 

 channel system; other minor changes are chiefly in the nature of 

 growth. But such considerations are beyond the scope and pur- 

 pose of this paper, and a discussion of them will be postponed 

 until a later date. 



