EMBKYOLOGY. 1 



By E, A. SCHAFER. 



FORMATION OF THE BLASTODERM. 



STEUCTURE OF THE OVUM AND CHANGES PKIOB TO SEGMENTATION. 



THE human body with all its tissues and organs is the product of the development 

 of a single nucleated cell, the egg-cell, germ-cell, or ovum, which is formed within 

 the principal reproductive organ of the female or ovary. The commencement of 

 development is preceded by certain changes in the ovum, which usually occur soon 

 after its discharge from the ovary, and consist (1) in the emission of certain 

 constituents of the nucleus which form the so-called polar globules ; (2) in the 

 accession of the nucleus of a sperm-cell or spermatozoon, which is formed within the 

 reproductive organ of the male (testicle), and which, blending with the remaining 

 part of the nucleus of the ovum, appears to take the place of the part which was 

 discharged in the form of the polar globules. 



An account of the structure of the ovum, and of the manner in which the above 

 changes are effected, may therefore appropriately precede the description of the 

 actual course of development of the ovum. 



Structure of the ovarian ovum. The human ovum resembles that of all other 

 mammals (with the exception of monotremes) in its minute size. Immediately 

 before the time of its discharge from the Graafian follicle of the ovary in which it 

 has been formed, it is a small spherical vesicle measuring about T^th inch ('2 mm.) 

 in diameter, and is just visible as a clear speck to the naked eye. When it is 

 examined with the microscope, it is found to be invested by a comparatively thick, 

 clear covering. This, when the centre of the ovum is exactly focussed, has the 

 appearance in optical section of a clear girdle or zone encircling the ovum (fig. 5), 

 and was hence named zona pellucida by von Baer (1827). But on more careful 

 examination with higher magnifying powers, and especially by the examination of 

 sections, there is not much difficulty in making out the existence of strise passing 

 radially through the membrane (fig. 6, zp). On this account, and especially since ;i 

 similar radially striated membrane forms a characteristic part of the investment of 

 the ovum in many animals belonging to widely different classes, ifc is more convenient, 

 in place of the name zona pellucida, which has been exclusively used to designate 

 this investment in mammals, to employ the more general term zona radiata, or to 

 speak of it simply as the striated membrane of the ovum. 



The zona radiata of the mammalian ovum is sufficiently tough to prevent the 

 escape of the contents of the ovum, even when subjected to a considerable amount 

 of pressure. If, however, the pressure be excessive, the tunic splits, and the soft 



1 It is mainly owing to the researches of His, published principally in the important monograph 

 " Anatomic menschlicher Embryonen " (Leipzig, 1880-1885), that our knowledge of the development of 

 the human embryo is now far more complete than was the case when the last edition of this work was 

 undertaken, and we are therefore able to keep more closely than was before possible to the human 

 species in following the course of development of the ovum. For the elucidation, however, of many of 

 the details of development, especially in its earlier stages, it will still be necessary to refer continually 

 to facts which have been made out only from the study of the embryology cf other mammals, as well aa 

 birds, reptiles, fishes, and even invertebrata. 



