1890.] The Mesoderm and the Colom. 895 
then the part where concrescence is finished. This concords 
with the observed fact that the head-process grows atthe expense 
of the primitive streak, as it would do if concrescence continued. 
The So-called Parablastic Nuclei of the Yolk—In meroblastic ver- 
tebrate ova, after the embryo is formed, there appear in the yolk 
near its surface, underneath the extra embryonic blastoderm, 
peculiar large nuclei, which are commonly designated as the para- 
blastic nuclei. The following description applies to Pristurus. The 
extra embryonic ectoderm is a rather thin, much-flattened epithe- 
lium lying close to the yolk; below the ectoderm is the super- 
ficial layer of the yolk, a broad stratum of protoplasm with 
scattered small yolk-granules; a little deeper down a row of 
irregular vacuolar spaces, and again, a little deeeper, a layer of very 
big nuclei, each with a distinct intra-nuclear network and several 
deeply stained nucleoli; the nuclei vary in size, being from 2-5 
times the diameter of the nuclei in the embryo. The upper part 
of the protoplasmatic stratum contains numerous small and a few 
larger yolk-grains, and contains near and under the embryo small 
nuclei; the middle part of the stratum contains the vacuoles, the 
big nuclei, and but few yolk grains; the deepest part contains 
larger granules, and merges gradually into the yolk proper (see 
also His, 78, 75, and Rückert, 32). Rückert designates these 
nuclei as “Merocytenkerne,’ and the cells which they represent as 
“Merocyten” The special function of the protoplasmic layer 
appears to be the assimilation of the nutritive yolk. Rickert 
also maintains, but without proper evidence, it seems to me, that 
merocytes become cells, some of which join the ectoderm, some 
the entoderm, and yet others the mesenchyma, In the Saurop- 
sida we find similar nuclei and similar relations of the nucleated 
layer, but in this type the protoplasmic layer becomes the epithelium 
of the yolk (see especially H. Strahl, 40), and I consider it prob- 
able that these parablast nuclei in all meroblastic ova belong to 
the vitelline entoderm. 
In holoblastic mammalian ova the vitelline entoderm is cellular, 
and no nuclei are known similar to the large “parablastic” 
nuclei of mesoblastic ova. 
5 From sections in the collection of Prof. His, which he generously permitted me to 
