FORMATION OF THE LAYERS. 265 



and receives none of its elements from the epiblast. In connec- 

 tion with its development, as two independent lateral masses, 

 I may observe, as I have previously done \ that in this respect 

 it bears a close resemblance to mesoblast in Euaxes, as de- 

 scribed by Kowalevsky 2 . This resemblance is of some interest, 

 as bearing on a probable Annelid origin of Vertebrata. Kow- 

 alevsky has also shewn 3 that the mesoblast in Ascidians is 

 similarly formed as two independent masses, one on each side 

 of the middle line. 



It ought, however, to be pointed out that a similar bilateral 

 origin of the mesoblast had been recently met with in Lym- 

 nseus by Carl Rabl 4 . A fact which somewhat diminishes the 

 genealogical value of this feature in the mesoblast in Elasmo- 

 branchs. 



During the course of this stage the spherules of food-yolk 

 immediately beneath the embryo are used up very rapidly. As 

 a result of this the protoplasmic network, so often spoken of, 

 comes very plainly into view. Considerable areas may some- 

 times be seen without any yolk-spherule whatever. 



On PI. 7, fig. 7 a, and figs. 11 and 12, I have attempted to 

 reproduce the various appearances presented by this network : 

 and these figures give a better idea of it than any description. 

 My observations tend to shew that it extends through the whole 

 yolk, and serves to hold it together. It has not been possible 

 for me to satisfy myself that it had any definite limits, but on 

 the other hand, in many parts all my efforts to demonstrate its 

 presence have failed. When the yolk-spherules are very thickly 

 packed, it is difficult to make out for certain whether it is present 

 or absent, and I have not succeeded in removing the yolk- 

 spherules from the network in cases of this kind. In medium- 

 sized ovarian eggs this network is very easily seen, and extends 

 through the whole yolk. Part of such an egg is shewn in PI. 7, 



1 Quart. Journ. of Microsc. Science, Oct., 1874. [This Edition, No. V.] 

 a "Embryologische Studien an Wiirmern u. Arthropoden." Mtmoires de V Acad. 

 S. Petersbourg. Vol. xiv. 1873. 



3 Archiv fiir Mikr. Anat. Vol. vn. 



4 Jenaische Zeitschrift, Vol. ix. 1875. A bilateral development of mesoblast, 

 according to Professor Haeckel (loc. cit.}, occurs in some Osseous Fish. Hensen, 

 Zeti.fitr Anat. u. Entw. Vol. I., has recently described the mesoblast in Mammalia 

 as consisting of independent lateral masses. 



B. 18 



