238 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
As to tlie manner in which Che keel is formed, Gotte’s description (IG) covers the 
ground pretty thoroughly. The lateral parts of the axeuj)latte, or the “ medullar 
platteu,” as Gotte calls them, crowd toward the median line. Here the Zellen- 
massen uach unten ausweichen, und die Axenplatte so gewissermasseu in derselben 
Eichtung eine geschlosseue Falte schliigt, was auch durch die vergangliche, oberflach- 
liche Furche angedeutet wird.” (Gotte, 16, quoted from Hoifmanu, 17, p. 21.) In the 
Figs. 53, 54, 57, and 58 the direction of cell movement in the region of the keel is easily 
discerned from the cell outlines, and is indicated by the arrows. Cell multiplication 
appears to play no part in forming the keel, which is produced from a mass of pre^ 
formed material by “ Zelleuverschiebuug.” As may be gathered from the quotation,’ 
Gotte believes that the “ Zellenverschiebirng,” by which the keel (which equals a' 
“geschlosseue Falte”) is built up, represents the invagination by which in most ver- 
tebrates the medullary groove is produced. I fully agree with him in this view of the 
process. The peculiarity of the Teleost in this respect may perhaps be expressed as 
follows: In the Teleost the cells which are destined to form the medullary cord are' 
precociously developed in the requisite numbers (axeui)latte). When the time conies 
to form the cord, the preformed cells move into their destined places, following in the 
main lines of movement (see arrows in Figs. 53, 54, 57) which, in the ancestor indicated 
the iiath along which the floor of the medullary groove traveled in the course of its 
m. 
Fig. 6. 
Diagram to illnstrate formation of invagiuate groove, 
as it is supposed to take place according to the “principle 
of unequal growth.’’ 
ITl deepening. In the ancestor, these lines of 
movement (see Fig. 6, arrows b) were (ac- 
cepting for the moment the view of His? 
and Hertwig) the resultants of the com-j 
biued forces produced by cell multiplica-| 
tion taking jilace in direction of arrows a.^ 
In the Teleost, the causal connection be-* 
tween cell multiplication and direction of I 
movement is not present, but the direc-* 
tion of movement itself has been retained. ^ 
It is impossible here not to be tempted into carrying the comparison a little fur-* 
ther, in the hope of getting some light thrown on the underlying principle of certain | 
embryonic processes. One of the apparently simplest of such processes is seen when , 
a unicellular membrane develops, by the method of invagination, a pit or a groove — 
as, foi' instance, in the development of an invaginate gastrula, or in the formations 
of an ideally simple neural groove. This phenomenon has received a well-known 
explanation, the strongest exponents of which are His and Hertwig. The princqfle 
involved in this explanation scarcely needs a description after Hertwig’s name 
for it has been given. It is called by Hertwig (Lehrbuch, p. 58) “ das Princip 
des ungleichen Wachsthums,” and is briefly this : Suppose the membrane in the 
diagram (Fig. G) to be composed of a single layer of cells. Now let rapid cell 
multiplication at right angles to the surface be set up over the area included 
between m and m. Pressure is exerted in the direction of the arrows, a, but meet- 
ing the lateral parts of the membrane which are not suffering cell multiplication, 
and hence act as rigid barriers, the effect of the pressure is first to form a groove, and 
then as the number of cells, and consequently the extent of surface, continues to in- 
crease, to drive the groove in the direction of arrows b. The mechanical simplicity of 
the explanation readily explains its popularity, and if true, its importance can scarcely .j 
