136 PHYLUM ECHINODEBMATA. 



endoderrnal pole of the blastosphere or of the invaginating endoderm, etc., 

 is really only a proliferation of nuclei, and that the migration of cells into 

 the blastocoel is nothing more than the shifting of these nuclei along the 

 strands of the protoplasmic reticulum which has traversed the blastocoel 

 from its first appearance. On this view the embryo must be regarded 

 not as an aggregation of separate cells, but as a continuous structure with 

 different densities of protoplasm in different parts. Thus in the blasto- 

 sphere stage the bulk of the protoplasm is concentrated in a peripheral 

 layer to which the nuclei are confined, the central portion being occupied 

 by a sparse non-nucleated reticulum. As to the meaning of this peculiar 

 peripheral aggregation of the protoplasm and nuclei, which is characteris- 

 tic of the blastosphere of so many pelagically developing animals, we are 

 quite in ignorance. It has been suggested that it represents some ances- 

 tral stage of structure common to all the organisms in which it occurs. 

 On the other hand it may be argued, perhaps with greater force, that 

 the condition is due to some physiological necessity, possibly of a nutri- 

 tive nature, which is felt by all organisms developing in the open sea. 



The undoubted fact of the presence of free leucocytes in the fluids of 

 the body must not be held to be at variance with the view of the nature 

 of the mesenchyme mentioned above. The view of the essential continuity 

 of the mesenchyme is based upon Comparative Embryology. In all 

 cases in which its structure has been thoroughly made out, it has the 

 form of a network continuous both with ectoderm and endoderm. The 

 free leucocytes which are found in the body fluids are products of this 

 network, formed no doubt for a definite purpose. What that purpose is 

 has been largely explained by the remarkable and penetrating observations 

 of Metschnikoff,* though the causes which govern the budding off of the 

 leucocytes have not been ascertained. 



The eggs when laid are enclosed in the vitelline membrane 

 (Fig. 87), but they soon acquire a coat of cilia and become free. 

 The gastrula has a somewhat oval form with the blastopore at 

 one end and one surface slightly flattened. The end carrying 

 the blastopore is posterior and the flattened surface is ventral. 

 The archenteron now bends towards the ventral surface 

 anteriorly, where, its wall having fused with the ectoderm, it 

 acquires an opening to the exterior ; this is the larval mouth, 

 the posterior opening or blastopore persisting as the larval anus 

 (except in Crinoids). 



The fate of the larval mouth and anus varies in the different classes. 

 In Holothuroidea they usually persist into the adult. In Ophiuroids the 

 larval mouth persists but the anus closes, the adult being without one. 

 In Asteroids and Echinoids the larval mouth and anus close and the 

 oesophagus atrophies, the adult mouth, oesophagus, and anus being new 

 formations. It is possible, however, that in some Asteroids they may 

 persist. In Crinoids there is no larval mouth or anus, and the blastopore 

 closes at an early stage. 



* See the numerous papers by this author dealing with the importance 

 of the amoeboid cells in the organism and especially his great work on 

 Immunity, English Translation, Cambridge, 1905. 



