300 A. A. W. HUBRECHT. 
increase in number (T77., figs. 44 and 53), and instead of form- 
ing compact masses they arrange themselves into radial and 
horizontal partitions, in which the flow of maternal blood is 
distributed. The size of the lacune with maternal blood- 
corpuscles immediately contiguous with blastocystic tissues is 
not in this way reduced as would appear at first sight from the 
description here given. On the contrary, we see that very 
large lacune persist (fig. 53, Sp.), but that at the same time 
the thickening which the trophoblast undergoes in conse- 
quence of the proliferating process here alluded to, leads to 
the replacement of the unicellular layer (77. of figs. 43, 51, 
and 52) by a region of very different aspect (44 and 53, Tr.), 
which it would be difficult to derive from that more simple 
starting-point, but for all the intervening stages which I have 
at my disposal, though they are not all here figured. 
Simultaneously with this histological change which the 
trophoblast undergoes, and with the enlarged capacity of its 
spaces, sp., for the reception of maternal blood, a more or less 
stratified layer of fusiform flattened cells is seen to be inter- 
posed between the trophoblast and the modified maternal 
tissue. This layer is marked s/. in figs. 44, 53, 54, &e. 
I would reserve my opinion in how far any embryonic elements 
enter into the composition of this layer, and cannot decide 
whether it is wholly embryonic or wholly maternal, or whether 
it is of mixed origin. The fact is that this layer coincides 
with the boundary line between embryonic epiblast and 
maternal decidual tissue, i.e. an imaginary line in the tropho- 
sphere,! which is on purpose not indicated in any of the dia- 
grams of Pls. XVIII and XIX, in which the colour is sufficiently 
indicative of such a fusion between grey maternal tissue and 
black embryonic trophoblast. The changes here more fully 
considered go on in the trophoblast of the upper half of the 
blastocyst; along the lower surface, where the mesoblast has 
not yet penetrated between tropho- and hypoblast, the former 
retains the aspect of the earlier stages above referred to. 
1 This name will be more strictly defined further on (p. 322); the region 
indicated by it includes both maternal and embryonic tissue. 
