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 segmentation is found in species in most groups of the animal kingdom. It is very common in Sponges and Coelenterates. 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_, _Chætonotus_, some Gephyrea (_Phoronis_); though not usual it occurs amongst Chætopoda, 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_[36]. [36] In the Rabbit and probably other Monodelphous Mammalia the segmentation is nearly though not quite regular. 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 protoplasm is necessarily most concentrated, and we can lay down as a general law[37] that the velocity of segmentation in any part of the ovum is roughly speaking proportional to the concentration of the protoplasm there; and that the size of the segments is inversely proportional to the concentration of the protoplasm. Thus the segments produced from that part of an egg where the yolk-spherules are most bulky, and where therefore the protoplasm is least concentrated, are larger than the remaining segments, and their formation proceeds more slowly. [37] _Vide_ F. M. Balfour, "Comparison of the early stages of development in Vertebrates." _Quart. Jour. of Micr. Science_, July, 1875.