22 



HUMAN ANATOMY. 



trary, the deutoplasm, or nutritive material, is collected towards one pole of the egg, 

 while the protoplasm, or formative material, is limited to the other ; eggs in which 

 these conditions obtain possess a distinctly polar yolk, and hence are known as 

 telolecithal ova. These aggregations of the protoplasm and the deutoplasm con- 

 stitute respectively the formative and the 7uitritive yolk, and correspond in position 

 to the a?mual and the vegetative poles of the egg. In an additional class of eggs, 

 the ce7it7'oIecithal, the yolk occupies the centre of the ovum, being covered by a 

 peripheral zone of formative material ; since such ova belong alone to certain in- 

 sects and are not found among vertebrates, they possess limited interest to students 

 of mammalian forms. 



Comparison of the behavior of these various groups of ova during segmen- 

 tation shows that only eggs poor in deutoplasm, as the alecithal mammalian and 

 amphibian ova, undergo complete cleavage during segmentation, those of the bird, 

 reptile, and fish undergoing cleavage only within the formative yolk. Ova, there- 

 fore, are classified according to the completeness of their division into those exhibit- 

 ing complete segmentation and those undergoing partial segmentation ; the former 

 are known as holoblastic, the latter as meroblastic. The embryologist further recog- 

 nizes an equal and an imeqiial complete segmentation according to the equality or 

 inequality of the cells, or blastomeres, resulting from the division of the ovum. 

 Since the segmentation spheres derived from the mammalian egg may be regarded 

 as practically of equal size, the egg of this class of animals, including the human 

 ovum, is described as an homolccithal holoblastic ovum, undergoing equal segmenta- 

 tion. It must be understood, however, that even in the sesfmentation of such ova 



Fig. 19. 



Fig. 18. 



Outer cells 



Zona pellucida 



Inner cells 



.Trophoblast 



— Ectoblast 



Entoblast 



Diagram of early mammalian blastodermic vesicle, 



consisting of trophoblast and inner cell-mass. 



{After I a?i Beneden.) 



-Trophoblast 



Zona pellucida 



Diagram of mammalian blastodermic vesicle ; inner 

 cells differentiating into ectoblast and entoblast. (After 

 Van Beneden.) 



the blastomeres very early exhibit inequality in size and in rapidity of division (Fig. 

 16), the effect of this differentiation being, that the more rapidly multiplying blas- 

 tomeres are smaller than the more slowly dividing elements. It is of interest, in this 

 connection, to note that the purest type of total equal segmentation is observed in 

 the ovum of the lowest vertebrate, the amphio.xus, — an animal whose de\'elopment 

 has shed much light on many obscure problems in the embryology of the higher 

 forms, including mammals and even man. 



The meroblastic bird's egg, on the contrary, undergoes cleavage only within a 

 limited circular field at its animal pole ; it is said, therefore, to undergo partial dis- 

 coidal segmen\.?it\on. In contrast to this, the centrolecithal ova exhibit partial super- 

 ficial segmentation, the peripheral zone of formative material alone undergoing 

 cleavage. 



The Blastoderm and the Blastodermic Layers. — The completion of 

 segmentation in holoblastic ova results in the production of d mass of blastomeres. 

 which is a solid sphere composed of mutually compressed segmentation cells ; to this 

 sphere the older anatomists gave the name of the morula, or the mulberry mass. 

 The solidity of the morula is temporary, since a cavity is soon developed within it. 

 This cavity, often called the segmentation cavity, increases to such an extent that a 



