THEORY OP THE EMBRYONIC STAGE OF ONTOGENY. 341 



elements in the blastophere were alike in structure and func- 

 tion. Later, however, coincidently with its acquiring the 

 capacity for moving in a definite direction, a change would take 

 place : the form first became elongated, and it is interesting to 

 observe that the free-swimming blastulas of both Echino- 

 cyamus and Eudendrium have this form ; then the cells at the 

 posterior end, being least favorably situated with regard to 

 promoting the locomotion of the colony, and best situated for 

 seizing food particles, since they are in a kind of backwater 

 from the eddies produced by the ciliary motion of the rest, 

 would become specially digestive ; increase in their number 

 could only take place coincidently with invagination if the 

 form of the colony were to be preserved and at the same time 

 the digestive cells were to remain in contact with the sur- 

 rounding medium — and thus we have the archenteron formed. 

 The cells at the anterior end, on the contrary, are in the best 

 position for receiving stimuli from the outer world ; and here 

 we should expect the first sense-organ to appear, and it is just 

 at this spot that we find the larval sense-organ of Conatula 

 with its associated nervous tissue, and the still more primitive 

 sense-organ of the Echinocyamus larva, this latter consisting 

 of a thickened patch of ectoderm bearing stiflF cilia, which take 

 no part in locomotion. In the same place the apical plate or 

 larval brain of the Trochophore is found, also bearing cilia or 

 more probably sense hairs. 



Metschnikoff's ' great objection to regarding invagination as 

 the primitive method of forming the endoderm was that the 

 blastopore sometimes became the mouth and sometimes the 

 anus. Sedgwick's suggestion, however, that mouth and anus 

 were differentiated from a slit-like blastopore, seems to answer 

 this difficulty. That a slit-like opening can be represented 

 by two independent perforations is shown by Echinoderm 

 development. Thus in Holothurians the larval mouth by a 

 shift of position becomes the adult ; in Asterids and Echinids, 

 on the other hand, it is represented by a totally new perfora- 



1 " Vergleichend Embryologisciie Studien. (3) Uber die Gastrula 

 Einiger Metazoen," ' Zeit. fiir wiss. Zool.,' Bd. xxxvii. 



VOL. 37, PART 3. NEW SBK. Z 



