PKIMITIVE STREAK. 



115 



of the gastrula has hitherto only been observed in some hydroid 

 Medusse (Geryonia). 



Finally, when the inequality of the segmentation is very pro- 

 nounced, the gastrula is formed by a process known as epibole. In 

 this process of development the epiblast cells, which are early distin- 

 guishable from the much larger hypoblast cells, spread themselves 

 over the latter as a thin layer (fig. 110); and in this, as in the 

 second method of development of the gastrula, the cavity of the 

 gastrula is, as a rule, a secondary formation in the centre of the 

 closely-packed mass of hypoblast cells. The blastopore is usually 

 found at the point where the complete enclosure of the hypoblast 

 is effected. 



It sometimes happens that a part of the primary blastosphere is 

 developed more rapidly than the remainder, and gives rise to a 



A 



PIG. 110. A, Unequal segmentation of the egg of Bonellia ; , epibolic gastrula of the 



same (after Spengel). 



bilaterally-symmetrical stripe-like thickening placed on the dorsal or 

 ventral surface of the embryo. Frequently, however, such a germinal 

 or primitive streak is not developed, and the rudiment of the embryo 

 continues to develop uniformly. Formerly great importance was 

 attached to these differences, the one being distinguished as an 

 evolutio ex una parte, and the other an evolutio ex omnibus partihus. 

 It is not, however, possible to draw a sharp line between these two 

 methods of development, nor have they the significance which was 

 formerly ascribed to them, for closely allied forms may present great 

 differences in this respect according to the amount of food yolk and 

 the duration of the embryonic development. 



The Coelenterata, the Echinoderms, the lower Worms and Mol- 

 luscs, Annelids, and even Arthropods and Vertebrates (Amphioxus) 

 present us with examples of regular development of all parts of the 



