52 FORMATION OF THE LAYERS. 



scribed by Kowalevsky \ This resemblance is of some interest, 

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

 alevsky has also shown '^ 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 Lymnceus 

 by Carl Eabl ^ A fact which somewhat diminishes the genea- 

 logical value of this feature in the mesoblast in Elasmobranchs. 



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. IV. fig. la, and PI. V. 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 show 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 defi- 

 nite limits, but on the other hand, in many parts all my efforts 

 to demonstrate its presence have failed. When the yolk sphe- 

 rules are very thickly packed, it is difficult to make out for cer- 

 tain 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 

 ^gg is shown in PI. V. fig. 14. In full-sized ovarian eggs, 

 according to Schultz'*, it forms, as was mentioned in the first 

 chapter, radiating striae, extending from the centre to the peri- 

 phery of the ^^^. When examined with the highest powers, 

 the lines of this network appear to be composed of immea- 

 surably small granules arranged in a linear direction. These 

 granules are more distinct in chromic acid specimens than in 



1 Embryologische Studien an Wurmen u. Ai'tbropoden. Memoir es d. VAcad. 

 S. Petersbourg. Vol. xiv. 1873. 



2 Archiv far Mikr. Anat. Vol. vii. 



3 Jenaisclie Zeitsckrift. Vol. ix. 1875. A bilateral development of meso- 

 blast, according to Professor Haeckel {loc. cit.), occurs in some Osseous Fish. 

 Hensen, Zeit. fur Anat. n. Entw. Vol. i., has recently described the mesoblast 

 in Mammalia as consisting of independent lateral masses. 



* Archiv fiir Mikr. Anat. Vol. xi. 



