EMBRYOLOGY OF CLEPSINE. 227 



were placed in osmic acid at intervals of fifteen minutes. The 

 selection of three well-marked events as starting-points serves 

 to eliminate, to a certain extent, the error in time which would 

 otherwise be sure to occur, as a consequence of the varying 

 rapidity of the changes under different temperatures. 



Before passing to the history of these three periods, I will 

 call attention to the composition of the mature egg, and to an 

 important change in the germinal vesicle. 



(a) Composition of the Egg.— The ripe egg consists of three 

 parts, viz. membrane, yolk, and germinal vesicle. The yolk is 

 composed of two distinct parts — (1) protoplasm and (2) deuto- 

 plasm. The transparent protoplasm is the ground- substance of 

 the eiig, in which the deutoplasmic elements are imbedded. The 

 deutoplasm is the yolk-granules and yolk -spheres before men- 

 tioned. These nutritive elements, the smallest of which exhibit 

 a most lively Brownian movement when brought in contact with 

 water, are perfectly passive with respect to all the movements 

 which characterise the egg in this and the following periods of 

 its evolution. They are simply surplus food material, the most 

 of which serves the wants of a late period in the development. 

 A detailed and accurate description of these elements has been 

 given by Eathke (yi^lu). 



T//e Germinal Vesicle. — The germinal vesicle lies excentrically. 

 Treated with osmic acid and carmine, it assumes a lead-grey 

 shade, slightly stained with the carmine. The germinal 

 dot ("macula germinativa^') is sometimes wanting (fig. 60, c), 

 sometimes present as a mere heap of fragments (fig. 60, b). The 

 contents of the germinal vesicle in ficf. 60, c, seem to have 

 retreated from one side, leaving vacuole-like spaces, separated 

 from one another by very attenuated walls of the very fine 

 granular substance ('■' nucleo-plasm," van Beneden.) I am un- 

 certain whether this condition is an artificial production, or an 

 incipient stage in the formation of the reticuhim, said to be 

 characteristic of tiie ripe ovum. Though doubtful in this 

 particular instance, I am convinced that nuclei do pass through 

 the reticular condition.^ I have often met with the same in 

 certain stages of the cleavage (fig. 61). Hoffman (-j-j) hns 

 noted this stage in the ovarian egg of Clepsine. The in- 

 vestigations of Heitzmann (69, various tissues), Biitschli (y) 

 Nephelis,) Frommann (49, blood-cells), Schwalbe (149, gang- 

 lionic cells), Flemming (38, Unio and Anodonta; 39, connective 

 tissue, endothdium, muscle, nerve, cartilage, and epithelium), 

 Flemming (fVij) and Giard (^VV? Echinus miliaris), van Beneden 

 (tW)> Asteracanthion and rabbit), O. Hertwig {-^'-^ -o) Toxo- 



1 This reticuhim was, so far as I am a\iare, first describtd by Kleuicii- 

 b erg 111 liis well-known work ou 'Hydra' (|j). 



