6 EMBRYOLOGY OF THE STARFISH. 



know whether what we call the outer envelope of the egg of an Echino- 

 derm is homologous to the outer envelope of the egg of an Acaleph, of a 

 Polyp, or of AVorms, Insects, or Crustacea, or how far these envelopes are 

 found in the ovarian eggs of Mammals, Birds, Reptiles, and Fishes. And 

 before we can come to a satisfactory result as to the place in the egg 

 which the spermatic particles reach before changes can be observed to 

 take place in the yolk, the eggs of the different classes of Animals must 

 be carefully compared with reference to this point. The first phenome- 

 non which precedes any change in the egg is a rotary motion given to 

 the whole egg by the constant beating of the spermatic particles ; the 

 germinative vesicle disappears (PI. I. Fi(/. 2) soon after this, and next the 

 germinative dot (PI. I. Fi//. 3). The yolk has then all the appearance of 

 an egg which has undergone segmentation, and the yolk of which should 

 consist of innumerable small spheres. The yolk has the same granular 

 structure previous to segmentation which has usually been considered to 

 belong to it only after the segmentation is complete. [The phenomena 

 preceding segmentation, the structure of the yolk, the mode of forma- 

 tion of the Kichtung's-Blaschen, the manner in which the germinative 

 vesicle disappears, are subjects which since the preceding investigations 

 ■were made have all received considerable attention. The explanations 

 given of these points are therefore all subject to revision and to correc- 

 tion. Sec more particularly the papers by Ludwig, C. Semper, Lax- 

 KESTER, Hartwig, Fol, Auerbacii, Balfour; sundry Embryological Me- 

 moirs by E. Van Benedex, Composition de I'oeuf, 1870; Kowalewskt 

 A. Mem. Akad. St. Peters, XVI., 1871 ; Blutsciili, Die Eizelle ; Haeckel 

 E., Die Gastraca Thcorie ; Strassburger, Die Zelle.] The resemblance 

 between these two stages is still more marked in the eggs of Cteno- 

 phoroB, where the ratio between the diameter of the yolk and that of 

 tlie outer envelope is large, and in which the segmentation is carried 

 on until the whole yolk consists of such minute spheres that it is impos- 

 sible at first sight to distinguish an egg of a Ctenophorous Medusa, which 

 has undergone complete segmentation, from one in wiiich the segmenta- 

 tion has not even begun, after the germinative vesicle and dot have dis- 

 ajjpcarcd. 'J'lit> disappearance of the germinative dot is accompanied by 

 a separation of the yolk from tlie inner wi.ll of tiie outer envelope of 

 tlie egg (PI. 1. /•///. 8); this is tlie first step towards segmentation, and 

 tlio presence of such a marked interval would greatly facilitate the detec- 



