THE SEGMENTATION OF THE OVUM. 77 



and by it each of the previous segments is halved (fig. 39 B). In the 

 third segmentation the plane of division is horizontal or equatorial 

 and divides each of the four segments 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 Cceleiiterates. 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, Cftccto-notus, some Gephyrea (P/toronis) ; 

 though not usual it occurs amongst Chsetopoda, e. g. Serpula. It is tlie 

 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, 

 Podnra 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 occurs only 

 in Ainphioxus '. 



Most of the eggs which have a perfectly regular segmentation are 

 of a very insignificant size and rarely contain much food-yolk: in 

 the vast majority of eggs there is present however a considerable bulk 

 of food material usually in the form of highly refracting yolk 

 spherules. These yolk spherules lie embedded in the protoplasm of 

 the ovum, but are in most instances not distributed uniformly, 

 being less closely packed and smaller at one pole of the ovum than 

 elsewhere. Where the yolk spherules are fewest the active proto- 

 plasm is necessarily most concentrated, and we can lay down as a 

 general law 2 that the velocity of segmentation in any part of the ovum 

 is roughly speaking proportional to the concentration of the pro- 

 toplasm there; and that the size of the segments is inversely pro- 

 portional to the concentration of the protoplasm. Thus the segments 

 produced from that part of an egg where the yolk spherules are most 



1 In the Babbit and probably other Monodelphous Mammalia the segmentation is 

 nearly though not quite regular. 



2 Vide F. M. Balfour, " Comparison of the early stages of development in 

 Vertebrates." Quart. Jour, of Micr. Science, July, 1875. 



