404 PHYSIOLOGY OF THE INVERTEBRATA. 



holoblastic. Holoblastic segmentation occurs in the Mam- 

 malia, Batrachians, the lower Crustacea, Vermes, &c. ; but if the 

 segmentation is incomplete or involves only a portion of the 

 vitellus, so that the remaining portion may be utilised as 

 nourishment during the early stages of the development of 

 the embryo, it is termed meroblastic. Meroblastic segmenta- 

 tion occurs in the ova of Aves, Amphibia, Cephalopoda, the 

 higher Crustacea,, and the Insecta. 



The detailed description of the changes which occur ia the 

 ovum after fecundation belongs to embryology, consequently 

 it is beyond the province of this volume, which treats of the 

 functions of animals after birth. Nevertheless, we shall allude, 

 in passing, to the broad outlines of the development of the 

 fecundated ovum.* 



Besides the above-mentioned mode of sexual reproduction 

 (viz., that of the fusion of two different elements), there is 

 another mode termed conjugation, or the union of two similar 

 protoplasmic masses. These may be derived from different 

 parts of the same individual, or from two individuals of the 

 same species. The union of these similar masses ultimately 

 results in the development of a new organism. This mode 

 of reproduction occurs in some Protozoa. 



The Protozoa. 



The mode of reproduction in the Monera and Protoplasta, is 

 either by fission or by endogenous cell formation. In the 

 former the cell divides into two portions, each portion gives 

 rise to a perfect organism. In the latter mode of repro- 

 duction a cyst is formed, and within this the protoplasm of 

 the original cell divides into a number of segments which 

 ultimately rupture the, parent cell and escape as separate 

 individuals. 



* For further details on the subject of embryology the reader is referred 

 to Balfour's Treatise on Comparative Embryology ; and Foster and Balfour's 

 Practical Emhryoloyy. 



