MUSEUM OF COMPARATIVE ZOOLOGY. ELE 
it appears immediately subsequent to the formation of the protovertebre, 
e. g., in case of the Duck, as a band of mesodermal cells extending length- 
wise of the embryo, between the protovertebre and the lateral layers, 
somatopleure and splanchnopleure ; that at first it remains in histological 
connection with the protovertebre on the one side, and with the lateral 
layers on the other ; that in later stages, as shown by Sedgwick (’80), 
the intermediate cell-mass is at intervals entirely cut off from relation 
with the lateral layers, while at alternating intervals it remains con- 
tinuous with them and is penetrated by the body cavity; and that 
subsequently the structure is wholly disconnected from the lateral meso- 
dermic layers, and constitutes what has been designated the “ Wolffian 
blastema.” These three steps in the process of development of the 
intermediate cell-mass in the Duck are represented in the Chick, ac- 
ccording to Sedgwick’s (’80) account, on/y in the anterior region of the 
embryo, to about the twentieth somite; while posterior to this region 
the intermediate cell-mass is from the beginning independent of the 
lateral mesoderm, thus presenting, apparently, an abbreviation of the 
process which takes place in the anterior part of the trunk. 
The expression “ intermediate cell-mass” is also applied to the homolo- 
gous structure in Elasmobranchs. According to Balfour (78, pp. 108, 
109, 127, 128), in Pristiurus and Torpedo at the time when the third 
visceral cleft appears, the intermediate cell-mass arises as the result of a 
fusion of the somatopleure and splanchnopleure at about the level of the 
dorsal aorta, and immediately above the dorsal limit of the true body 
cavity. The mass of cells involved in this fusion Balfour regards as 
homologous to Waldeyer’s “ intermediate cell-mass ” in the Chick. He 
does not state how far forward in the trunk this fusion of the mesoder- 
mal layers extends, but points out that from it the urinogenital system 
is developed, and shows that the first trace in the development of this 
system is the beginning of the segmental duct, which appears as a solid 
knob in the region of the fifth protovertebra, and grows backward along 
the inner surface of the ectoderm as a solid rod of cells. 
An homologous structure is also present in Lacertilia. The accounts 
of the intermediate cell-mass in the Lizard by different observers agree 
in general, but differ somewhat in their details, and especially concerning 
its fate. As shown by Braun (’77) and Weldon (’83), it is present in 
the Lizard from the beginning of the segmentation of the mesoderm, 
and is connected both with the protovertebre and the lateral mesoder- 
mic layers. According to Weldon’s account, in an embryo of Lacerta 
muralis at a stage in which eleven protovertebre are formed, there 
