ORIGIN OF THE GERMINAL LAYERS. 341 



or anus appears to be due simply to the presence of a large 

 amount of food-yolk. The cases of the Cephalopoda, of Euaxes, 

 and perhaps of Clepsine and Bonellia, are to be explained in 

 this way : in the case of all these forms, except Bonellia, the 

 blastopore has the form of an elongated slit along the ventral 

 surface. This type of blastopore is characteristic of the Mollusca 

 generally, of the Polyzoa, of the Nematelminthes, and very 

 possibly of the Chaetopoda and Discophora. In the Chaeto- 

 gnatha (fig. 209 B) the blastopore is situated, so far as can be 

 determined, behind the future anus. In many Decapoda the 

 blastopore is placed behind, but not far from, the anus. In the 

 Chordata it is also placed posteriorly to the anus, and, 

 remarkably enough, remains, in a large number of forms, for 

 some time in connection with the neural tube by a neurenteric 

 canal. 



The great variations in the character of the gastrula, indicated 

 in the above summary, go far to shew that if the gastrulae, as we 

 find them in most types, have any ancestral characters, these 

 characters can only be of the most general kind. This may 

 best be shewn by the consideration of a few striking instances. 

 The blastopore in Mollusca has an elongated slit-like form, 

 extending along the ventral surface from the mouth to the 

 anus. In Echinodermata it is a narrow pore, remaining as the 

 anus. In most Chsetopoda it is a pore remaining as the mouth, 

 but in some as the anus. In Chordata it is a posteriorly- 

 placed pore, opening into both the archenteron and the neural 

 canal. 



It is clearly out of the question to explain all these differences 

 as having connection with the characters of ancestral forms. 

 Many of them can only be accounted for as secondary adap- 

 tations for the convenience of development. 



The epibolic gastrula of Mammalia (vide pp. 215 and 291) is 

 a still more striking case of a secondary embryonic process, and 

 is not directly derived from the gastrula of the lower Chordata. 

 It probably originated in connection with the loss of food-yolk 

 which took place on the establishment of a placental nutrition 

 for the foetus. The epibolic gastrula of the Scorpion, of Isopods, 

 and of other Arthropoda, seems also to be a derived gastrula. 

 These instances of secondary gastrulse are very probably by no 



