12 
The Embryology of Patella. 
slit-like cavity appears in the centre of the mass of endoderm 
ceils, the dorsal and anterior walls of whicli are formed by the 
larger ceils, the oldest and largest cells forming the anterior 
wall. The posterior ventral wall of the cavity is formed by the 
smallest and youngest ceils and also by those ceils which, with 
the closure of the blastopore, were forced out of the month of the 
gastrula into the interior of the erabryo. 
Before the blastopore has commenced to leave its primitive 
position opposite the apical pole, the endo-mesöderm ceils leave 
their median lateral position and approach each other on the 
future dorsal side of the embryo, at the same time they become 
elongated, the inner ends being rounded and somewhat distended, 
wliile their outer ends are more or less rod-shaped owing to the 
pressure of the adjacent ceils. When the cells in question have 
reached a distinetly dorsal position, they divide in a plane perpen- 
dicular to about tbe middle of the long axis of the cells, and thus 
give rise at their inner ends to the primitive mesoderm cells, which 
for some time show by their position and contour the source of 
their origin. A long time after the production of the primitive meso¬ 
derm cells , one may see in sections the stalk-like ends of the endo- 
mesoderm cells situated on the dorsal edge of the blastopore, hy the 
subsequent closure of which they are forced into the interior of the embryo, 
and form a part of the endoderm lining of the mesenteron. 
Düring the first movements of the blastopore I have often 
noticed two small cells between the pi'imitive mesoderm and the 
remnants of the endo-mesoderm cells. PI. III, Pig. 40 a. I think that 
they originated by a second division of the latter cells. 1 was not 
able to find them in later stages. Whether they are the products 
of abnormal development or not, I am unable to say but am con- 
fident that they do not take any part in the formation of the 
mesodermic chords. They disappear soon after their first appearance. 
The two primitive mesodenji cells, whose dorsal position 
now marks a further advance in the differentiation of the simple 
bilateral into the dorso-ventral type, are large and spherical, but 
finally become placed so closely to each other, that their point of 
contact is transformed into a plane, which exactly coincides with 
the median longitudinal dorso-ventral plane of the embryo. PI. III, 
Fig. 36, and PI. II, Fig. 29, and 30. This partially spherical form 
soon changes to an oval one that precedes the first division, which 
takes place at right angles to the long axis of the cell and 
(ICO) 
