DEVELOPMENT OF LYMPHATICS IN ANURA 109 
SUMMARY 
1. On the development of the primary maxillary lymph sinus 
The sinus begins in approximately 5-mm. embryos of Bufo 
vulgaris in the form of small discontinuous anlagen, which appear 
either as cellular thickenings of the endothelium of the develop- 
ing jugular veins or as islands lying in the mesenchyma in the 
immediate vicinity of these vessels. 
During development all vascular anlagen of the head region, 
both haemal and lymphatic, can be distinguished from the sur- 
rounding mesenchyma by the greater number of yolk globules 
present in their endothelium. 
The originally solid lymphatic anlagen acquire lumina, which 
have their inception as small crevice-like spaces in the cytoplasm 
between the large yolk globules. 
By continued proliferation and growth, the individual anlagen 
increase in length, bud collateral branches, coalesce with one 
another, and in time form a complex tubular network extending 
in a curved plane from the region of one external jugular vein to 
that of the opposite side; this network represents the principal 
or mandibular division of the primary maxillary lymph sinus. 
The other divisions, the circumoral, temporal, and pericardial, 
arise from the mandibular division by outgrowth and extension. 
The lymphatic network becomes transformed into a spacious 
and uninterrupted sinus by the progressive expansion of all the 
anastomosing channels and by the reduction and tearing of the 
intervening mesenchymal strands and trabeculae. 
During the preceding genetic stages, the sinus possesses no 
outlet; it is not confluent with the veins. The sinus receives an 
outlet in approximately 10-mm. embryos as the posterior pro- 
longations of its temporal divisions join the jugular lymphatics 
and thereby are placed in communication with the anterior lymph 
hearts and through them with the venous system. 
The extension and distention of the developing sinus are prob- 
ably achieved by the increasing internal pressure on its walls of 
the accumulating lymph before an exit is established. Duringthe 
expansion of the sinus, the lining cells become progressively 
flattened and assume typical endothelial qualities. 
