470 REIGHARD—PHELPS. [Vou. XIX. 
penser, and Lepidosteus. This study has necessarily involved 
some consideration of the head mesoblast. 
We shall distinguish two phases in the history of the ad- 
hesive organ: first a progressive phase which extends from 
the origin of the organ to a stage of the larva shortly after 
hatching, at which time the organ becomes functional, and 
second a retrogressive phase which extends from the close 
of the functional period to the time of the entire disappear- 
ance of the organ. In considering each of these phases we 
shall have occasion to refer to certain stages in the develop- 
ment of the embryo and larva, but shall describe these stages 
only in sufficient detail to render possible their subsequent 
identification. 
THE PROGRESSIVE PHASE. 
The first stage.* a. The Adhesive Organ. The first ex- 
ternal indication of the adhesive organ is found in embryos 
which still lie flat on the yolk (Plate, Fig. 1), with neither 
head nor tail protuberant. The brain at this stage shows 
externally two divisions. only. The limits of the hindbrain 
are not distinguishable, but near its anterior end it appears 
*This early stage of the adhesive organ has been found in surface views, 
only in embryos preserved after removal of the egg membranes or by a 
method which does not cause shrinkage of the membranes. If the eggs are 
preserved without such precaution shrinkage of the membranes brings 
them into contact with the surface of the embryo, obliterates many exter- 
nal features, changes topographic relations, and deforms certain internal 
structures. The sections figured by Eycleshymer and Wilson (1908, early 
stages) show the membranes still in situ and shrunk against the embryo. 
The use of such material probably accounts for their statement that the 
adhesive organ appears as a pait of diverticula. Shrinkage of the mem- 
branes obliterates the delicate “crescent” which is unpaired and includes 
the adhesive organ. This should appear in Fig. 13, Pl. I, of Eycleshymer 
and Wilson, 1906. They say that in embryos of ninety hours “the adhesive 
organs are not observable in surface views” (1908, p. 135), and they figure 
this stage without adhesive organ in Pl. I, Fig. 15, of their paper of 1908. 
On the contrary, we find the organ well developed and paired in surface 
views in a somewhat younger stage (cf. our plate, Fig. 2). Shrinkage of 
the membranes obliterates these delicate surface features in early stages, 
and gives rise to the impression that they appear only in later stages. 
