MATURATION OF THE OVUM. 961 



CLEAVAGE, MORULA, BLASTULA, GASTRULA, FORMATION OF 

 THE GERMINAL LAYERS. FIRST RUDIMENTS OF THE 



EMBRYO. 



In the fecundated ovum the yolk-mass contracts more closely 

 about [ ; the newly [formed nucleus, becoming somewhat separated 

 from the yolk-membrane, and there now follows division first of the 

 nucleus and then of the yolk into two nucleated globules or blast omeres. 

 This process, designated total cleavage, is repeated in accordance with 

 the method of cell-division in the- two globules formed, so that first 

 4, then 8, 16, 32, etc., globules result. The division ceases only 

 after the entire yolk has been subdivided into numerous small 

 globules, the nucleated cleavage-spheres, or the unencapsulated pro- 

 toplasmic primitive cells (from 20 to 25 /JL in diameter). The yolk 

 now consists of a collection of primitive cells and is designated the 

 morula or the mulberry mass. 



The division of the nucleus of the ovum takes place by mitosis after the 

 previous formation of a spindle-form. The centrosomes of this first cleavage- 

 spindle are derived from the centrosome of the spermatozoon. According to 

 the observations of van Beneden the constituents of both the male and the female 

 pronucleus pass over into the cleavage-spheres, so that all cells of the body are 

 formed from a combination of the male and female procreative elements. This 

 fact explains the process of inheritance from the paternal and the maternal or- 

 ganism. Deficiency of oxygen gives rise in the ova of some fish to an involution in 

 the process of cleavage. The globules become dissolved and coalesce. A renewal 

 of the supply of oxygen stimulates the process of cleavage anew. 



The uniform mode of cleavage described, such as occurs in mammals and 

 amphioxus, is designated equal or adequal. A second variety of cleavage is the 

 total unequal, in which, for example in the frog's egg, one-half of the yolk, 

 designated the animal pole, which often is pigmented, yields much smaller cleav- 

 age-cells than the other or vegetative pole. The embryo forms in the animal 

 pole. When, finally, the yolk-mass has become so large that cleavage remains 

 confined to the animal pole, then partial cleavage (described later) occurs. 



Numerous attempts have recently been made to trace the development from 

 a single isolated blastomere. Some investigators found that at first only a right 

 or a left half of an individual (echinodermata) is formed from one blastomere, 

 but that this in the further course is capable through post-generation to develop 

 into an entire being. Other observers, on the other hand, from the outset ob- 

 tained from a single blastomere (for example from an ascidian ovum) an entire 

 individual (even to the sixteenth division), but of smaller size. Under special 

 experimental conditions, finally, it was possible to produce double malformation, 

 from partially isolated, but partially connected blastomeres. 



Under normal conditions the first line of cleavage (frogs) passes, according to 

 Roux, in the same direction as the central nervous system. The second fissure 

 intersects the first at right angles and divides the ovum into two unequal parts, 

 of which the larger serves to form the cephalic portion of the embryo. 



Meanwhile the ovum has increased in size through the absorption 

 of fluid. All of the cells are polyhedral in shape from mutual pressure, 

 and form a cellular vesicle, the germinal vesicle, which is applied through- 

 out its periphery to the zona pellucida. 



The human ovum has reached this stage of development during the first 

 week; that of rabbits in 4 days, of guinea-pigs in 3^, of cats in 7, of dogs in n, 

 of foxes in 14, of ruminants and pachydermata in from 10 to 12, of deer in 60 

 days. In some animals (for example rabbits) the zona pellucida is further sur- 

 rounded by a layer of albumin. A small collection of blastomeres do not partici- 

 pate in the formation. They are apparently not utilized, and apply themselves 

 at one point on the interior of the blastula, and here, later, the embryo develops. 

 6x 



