72 AN INTRODUCTION TO ZOOLOGY. 



alteration and becomes, on the coritrary, the excretory 

 opening, or anus ; and sometimes it disappears, while 

 a new mouth and anus arise. ■ The Gastrula is usually 

 formed by invagination ^ in the manner described, bat 

 not always. 



The Morula and Gastrula stages are believed to 

 represent the stages passed through by the animal's 

 ancestors ; first a multicellular Stage with cells all 

 alike, then a free-swimming animal with a mouth and 

 stomach. The two-layered Gastrula may be compared 

 in structure to a Hydra (page 130, and Fig. 11, page 

 40) without tentacles. 



The Gastrula stage consists of the two primary 

 body layers. At a later stage the mesoblast is formed, 

 consisting of amoeboid cells (Fig. 10). It is derived 

 from the primary layers, but by processes which are not 

 the same in all cases. The lymph corpuscles, or white 

 corpuscles of the blood, may be considered to represent 

 the original type of mesoblast cell, very like a free 

 amoeba. Cells of this type compose the bulk of the 

 mesoblast in many of the lower invertebrata, and in 

 prepared and coloured sections of the animals are seen 

 as cells with long processes; in the living animal these 

 processes are capable of some movement. In the 

 lower forms of animal life the animal stops at this state 

 of development, the epiblast and hypoblast remain- 



' Invagination (from Lat. vugina, a sheath or hollow case) 

 is the term applied to the formation of a structure arising in 

 this manner as a fold in a previously continuous wall of 

 tissue. 



