422 A. A. W. HUBRECHT. 
primary epiblast has been disintegrated, I take it to be more 
probable that the greater part of it is carried off by the meso- 
blast cells, and plays a further part in the formation of the 
larva. 
The differences in the mesoblast cells, which were noticed, 
may be partly due to the different methods of preparation ; 
nevertheless, I will briefly describe them. Sometimes they 
have all a rounded shape with one distinct nucleus, strongly 
stained by carmine, sometimes more nuclei appear to be 
present, at least more elements which have the same affinity 
for carmine as the nucleus, and are often regularly arranged 
around a central nucleus. Sometimes I even noticed meso- 
blast cells which had all the appearance (l.c. (30), pl. ii, 
fig. 28) of containing faded nuclei, that had perhaps belonged 
to more than one cell,! in each of which a distinct nucleolus 
was visible. Other mesoblast cells contain small, strongly 
refractive bodies, whilst it may in conclusion be noted that 
the protoplasm of the mesoblast cells is not always similarly 
affected by picro-carmine ; sometimes remaining colourless, 
sometimes becoming light rose-coloured. I mention these 
different variations because I feel sure that they may be of 
considerable cytological importance, although for myself I have 
no leisure to consider them from that point of view in the 
present paper. 
The primary epiblast has one more function, which we have 
not as yet considered. It forms a paired invagination, one on 
each side of the blastopore, commencing as a shallow depres- 
sion (l.c. (30), pl. i, figs. 835—42 ; pl. ii, figs.46—48) which gra- 
dually deepens, finally closes up and is nipped off from the 
primary epiblast. A spherical sac with a central cavity thus 
appears in a certain stage of development on each side of the 
blastopore (Pl. XXII, figs. 4.and5). These sacs come to lie in 
the blastoccel and are only later on enclosed in the secondary 
epiblast at the time when this increases in size and coalesces 
(figs. 5, 6, 11). During this process they moreover change 
1 Dr. van Rees tells me that similar “ scavenger” cells were noticed by him 
during the process of the metamorphosis of the larva of Musca vomitoria. 
