94 REGULAR SEGMENTATION. 



When the process of regular segmentation is examined some- 

 what more in detail it is found to follow as a rule a rather definite 

 rhythm. The ovum is first divided in a plane which may be 

 called vertical, into two equal parts (fig. 39, A). This division is 

 followed by a second, also in a vertical plane, but at right angles 

 to the first plane, and by it each of the previous segments is 

 halved (fig. 39, B.) In the third segmentation the plane of divi- 

 sion is horizontal or equatorial and divides each of the four seg- 

 ments into two halves, making eight segments in all (fig. 39, C). 

 In the fourth period the segmentation takes place in two vertical 

 planes each at an angle of 45 with one of the previous vertical 

 planes. All the segments are thus again divided into two equal 

 parts. In the fifth period there are two equatorial planes one on 

 each side of the original equatorial plane, and thirty-two spheres 

 are present at the close of this period. Sixty-four segments are 

 formed at the sixth period, but beyond the fourth and fifth periods 

 the original regularity is not usually preserved. 



In many instances the type of segmentation just described cannot be 

 distinctly recognized. All that can be noticed is that at each fresh 

 segmentation every segment becomes divided into two equal parts. It is 

 not absolutely certain that there is not always some slight inequality in 

 the segments formed, by which, what are known as the animal and vegetative 

 poles of the ovum, can very early be distinguished. A regular segmen- 

 tation is found in species in most groups of the animal kingdom. It is 

 very common in Sponges and Ccelenterates. Though less common so 

 far as is known amongst the Vermes, it is yet found in many of 

 the lower types, viz. Nematoidea, Gordiacea, Trematoda, Nemertea 

 (apparently as a rule), Sagitta, Chcetoiwtus, some Gephyrea (Phoronis] ; 

 though not usual it occurs amongst Chaetopoda, e.g. Serpula. It is the 

 usual type of segmentation amongst the Echinodermata. Amongst the 

 Crustacea it appears (for the earlier phases of segmentation at any rate) 

 not infrequently amongst the lower forms, and even occurs amongst the 

 Amphipoda (Phronima). It is however very rare amongst the Tracheata, 

 Podura affording the one example of it known to me. It is almost as rare 

 amongst Mollusca as amongst the Tracheata, but occurs in Chiton and is 

 nearly approached in some Nudibranchiata. In Vertebrata it is most nearly 

 approached in Amphioxus*, 



Most of the eggs which have a perfectly regular segmentation 

 are of a very insignificant size and rarely contain much food- 



1 In the Rabbit and probably other Monoclelphous Mammalia the segmentation is 

 nearly though not quite regular. 



