106 bulletin: museum of comparative zoology. 



POSTSCRIPT. 



Since the above was written, Professor Salensky has had the kindness 

 to afford me an opportunity of examining his sections of the embryos of 

 Distaplia magnilarva. As these preparations show a class of facts, not 

 observed in mine, which is of much importance in considering the origin 

 of the cells in the test, their consideration is imperative. But before I 

 proceed to discuss the differences, I will mention two points in which 

 our preparations agree. 



1. What I have said about the flattening of the test cells against the 

 ectoderm (p. 86) applies equally well to those of Salensky's prepara- 

 tions that I have seen. The flattening appears to be entirely due to 

 the pressure of the embryonal membranes. Wherever the test cells 

 were flattened and the follicular membrane could be made out at all, 

 the membrane was seen to be pressed against the surface of the embryo ; 

 and, conversely, wherever the follicular membrane was seen to lie at 

 some distance from the ectoderm, as where it passes over the tail, the 

 test cells were never seen flattened against the ectoderm. 



2. The granular substance sometimes found between the test cells in 

 D. occidentalis is also present in D. magnilarva. It seems, in this case 

 also, to be an artifact, being likewise found within the cavities of the 

 embryo, and principally on one side of the embryo. But it is more 

 finely granular and less one-sided in its distribution in D. magnilarva 

 than in D. occidentalis. 



The diffei'ences between Salensky's preparations and mine are con- 

 nected with cells situated entirely outside the test-matrix, but having 

 a structure intermediate between that of the cells in the test and the 

 " test cells " (Salensky's kalymmocytes). Whereas, in my preparations, 

 I coidd find no intermediate stages, in Salensky's quite a perfect transi- 

 tion could be traced from cells that could not be distinguished with 

 certainty from typical kalymmocytes to those looking just like the 

 characteristic vacuolated cells of the test. But among the cells on the 

 outside of the test-matrix the series may be traced even farther, even 

 to the characteristic undifferentiated mesoderm cells with large nuclei 

 atid very scanty compact cytoplasm. These facts, and othei's to be 

 mentioned presently, have forced upon me the conclusion that in 

 D. magnilarva the mesoderm cells wander through the ectoderm, not 

 only into the test, but also on to the surface of the embryo, and there 

 undergo a degenerative vacuolation, the final result of which cannot be dis- 



