7 OTTO F. KAMPMEIER 
interrogation mark. It is probable that during the fixation of 
the embryo, the very slender connecting channel had collapsed 
or contracted into such a delicate strand that it became im- 
possible to distinguish it from the surrounding mesenchymal 
reticulum. 
The reconstruction (fig. 30) shows that the primary outgrowths 
of the circumoral, temporal, and pericardial divisions from the 
principal plexus keep close to haemal vessels, potential veins. 
In fact, frequently, and particularly in the case of the pericardial 
division, the lymphatic extension adheres to the wall of the 
blood vessel. The writer has been unable to decide whether or 
not, in the elongation of such lymph channels, the endothelium 
of the blood vessel contributes cells to the growing tip. It is 
conceivable that the latter might simply advance along a path 
of least resistance or in accordance with certain stresses or cur- 
rents that may closely parallel the blood vessel. As yet we are 
entirely ignorant of the presence or absence of any such pro- 
nounced currents in the tissue interstices before the advent of the 
haemal and lymphatic capillary systems, and the suggestion that 
the paths invariably taken by these primary lymphatic exten- 
sions may be predestined by the existence of definite antecedent 
streams, acting as a stimulus or directive force to the prolifera- 
ting endothelium, is pure conjecture. A cross-section of the 
pericardial division illustrating the adhesion of the lymph vessel 
to the haemal one is shown in figure 14 (lym. and v. jug. ext. sin.). 
During the formation of the plexus phase of the primary maxil- 
lary sinus, the sprouts and the most recently established anasto- 
moses are usually solid, the acquisition of lumina, however, 
occurring very soon. During this period, too, the number of 
yolk spherules in the lining cells are still very abundant. 
The next phase in the development of the lymph sinus is the 
transformation of the plexus into a spacious and uninterrupted 
chamber. This process is a rapid one, being practically finished 
in the embryonic period between 8- and 10-mm. stages (B. vul- 
garis). The genetic changes consist in the progressive expan-: 
sion of all the anastomosing channels, so that the gaps in the net- 
work are reduced and the mesenchyme filling them is compressed 
