THE MICROSCOPE. 



Vol. V. ANN ARBOR, FEBRUARY, 1885. No. 2. 



ORIGINAL COMMUNICATIONS. 



OUTLINES OF EMBRYOLOGY. 



SECOND PAPER. 



h. Impregnation. 



THE maturation of the ovum and the formation of the polar glo- 

 bules may take place either before or after the impregnation, 

 though in logical sequence it is really first in order. In the 

 light of the previous section we see that impregnation is really 

 the replacement of that which was lost, and in such a way as 

 usually to ensure cross-fertilization, which has been shown to 

 be highly beneficial to the race. We need but to allude to the 

 investigations of Darwin and Hermann Midler in this connec- 

 tion, and although their labors were almost entirely confined to 

 the vegetable world, the same circumstances and necessities 

 exist here. 



In discussing the phenomena connected with impregnation 

 we have again to return to the investigations of Hermann Fol 

 on the echinoderms as a basis for our studies. Dr. Fol has a 

 peculiar compressor which he uses in his investigations, which 

 is admirably adapted for its purposes. Without attempting to 

 describe it now, we may mention its advantages and the meth- 

 ods of using it. The eggs of the star-fish were taken from the 

 parent and placed with sea-water between the two glasses of 

 the compressor, which were brought together as closely as pos- 

 sible without injury to the eggs. Some of the milt was then 

 taken from a male star and placed in the compressor through a 

 hole in the upper glass, the eye being at the same time applied 

 to the microscope. 



Very soon the spermatozoa were seen to enter the field and 

 approach the egg. Around the egg is a thick layer of mucus, 



