The Embryology of Patella. 
13 
nearer the anterior than the posterior end, thus giving rise to 
two uneqnal parts. PI. IV, Fig. 49 and PL III, Fig. 37. 
Before the nuclear changes which precede division have 
appeared, the nucleus is surrounded by an envelope of finely 
granulär protoplasm, which, just previous to the formation of the 
spindle, becomes beautifully radiate sending long thread like arras 
of protoplasm toward the periphery of the cell, which is filled 
with the vacuolated protoplasm similar to that seen in the endo¬ 
derm cells; PI. II, Fig. 32. PL IV, Fig. 49 represents a section 
through one of the primitive mesoderm cells during its first division ; 
cells m 1 and m 2 and the subseqnent cells produced from pr. m do not 
divide themselves until each of the primitive mesoderm cells has 
given rise to a single row of smaller cells, the two V shaped rovvs 
thusformed divergingforward and downward, whilethetwo primitive 
mesoderm cells form their point of convergence; PL IV, Fig. 50. 
The smaller mesoderm cells now divide giving rise to a double 
row of cells compactly arranged and extending forward beyond 
the velum. The primitive mesoderm cells are even then plainly 
visible as two large cells at the point where the mesoblastic 
cords meet. 
General shape. During the changes just described the 
general shape of the embryo has undergone some rather important 
modifications to which we will now turn our attention. The basal 
end of the embryo was at first round, but by the time the blasto- 
pore has reached the edge of the ventral surface, it has become 
dorso-ventrally compressed; PL II, Fig. 30 and 32. As the blasto- 
pore encroaches still more upon the ventral surface two swellings 
may be observed on each side of it; Pl. II, Fig. 31. As the blasto¬ 
pore moves forward and leaves its intermediate posit'ion hetween the 
two swellings , they unite and form a median protuberance which deve- 
lopes into the foot, wliose dual origin is for a long time indicated by 
the presence of a small median indentation. 
When the blastopore closes the foot has only attained in- 
significant proportions. 
After having obtained an idea of the general form, and the 
relation of the internal to the external changes which have 
taken place, we can more easily comprehend the exact nature of 
the movements of the blastopore, which have thus far played so 
important a part and served as a landmark for the study of the 
other changes which take place. As we might very well imagine, 
a priori, the motive force of the ventral movement of the blasto- 
(161 
