84 



PARTIAL SEGMENTATION. 



Many eggs, such as those of the Myriapods 1 , present an irregular 

 segmentation; but the segmentation is hardly unequal in the sense in which 

 I have been xising the term. Such cases should perhaps be placed in the 

 first rather than in the present category. 



The type of unequal segmentation is on the whole the most widely 

 distributed in the animal kingdom. There is hardly a group without 

 examples of it. 



It occurs in Porifera, Hydrozoa, Actinozoa and Ctenophora. Amongst 

 the Ctenophora this segmentation is of the most typical kind. Four equal 

 segments are first formed in the two first periods. In the third period a 

 circumferential furrow separates four smaller from four larger segments. 



This type is also widely distributed amongst the unsegmented (Gephyrea, 

 Turbellaria), as well as the segmented Vermes, and is typical for the 

 Eotifera. It appears to be very rare in Echinoderms (Eckinaster Sarsii). 

 It is not uncommon in early stages of the segmentation of the lower 

 Crustacea. 



For Mollusca (except Cephalopoda) it is typical. Amongst the Ascidia 

 it occurs in several forms (8alpa, Molgula) and amongst the Crauiata it is 

 typical in the Cyclostomata, Amphibia, and some Ganoids, e.g. Accipenser. 



Partial segmentation, The next type of segmentation we have 

 to deal with has long been recognized as partial segmentation. 

 It is a type in which only part of the ovum, called the germinal 

 disc, undergoes segmentation, the remainder usually forming an 

 appendage of the embryo known as the yolk-sack. Ova belonging 

 to the two groups already dealt with are frequently classed together as 

 holoblastic ova, in opposition to ova of the present group in which the 

 segmentation is only partial, and which are therefore called meroblastic. 

 For embryological purposes this is in many ways a very convenient 



FIG. 44. SURFACE VIEWS OF THE EARLY STAGES OF THE SEGMENTATION IN A FOWL'S EGG. 



(After Coste.) 



a. edge of germinal disc. b. vertical furrow, c. small central segment, d. larger 



peripheral segment. 



classification, but ova belonging to the present group are in reality 

 separated by no sharp line from those belonging to the group just 

 described. 



1 Metschnikoff. Zritanlirift f. wiss. Zooloaif. 



