MATURATION OF THE OVUM. 



pellucida (Z), and the hypoblast (h). The primitive mouth (u) here 

 also leads to the central cavity of the ovum. When the rabbit's ovum 

 has reached a diameter of 2 mm. there appears at one point the 

 oval embryonal spot or embryonal shield (germinative or embryonal 

 area). The cells of the ectoderm multiply so as to form several layers 

 in the region of the shield. Careful examination leads to the detection 

 further at the border of the latter of a small longitudinal area (//, u), 

 from which the duplication of the cell-layer of the blastula takes place, 

 and which, therefore, must be looked upon as the blastopore. From 

 the blastopore the lower layer of cells (hypoblast) extends in the region 

 of the embryonal spot, although its growth continues uninterruptedly, 

 until finally the entire blastula consists of two layers. The site of the 

 primitive mouth (//, u) becomes the so-called primitive streak (///, pr), 

 which at first appears as an oval elevation, and later as a longitudinal 

 furrow. 



The primitive streak (like the primitive mouth in general in vertebrates) 

 is a temporary structure. It is, however, still present when the medullary groove 

 is formed in the epiblast (IV, rf) in front of it; then it gradually atrophies. This 

 subject will later on be discussed at greater length. The primitive streak presents 

 a nodular swelling (Hensen's nodule) anteriorly, and posteriorly a terminal en- 

 largement. The furrow of the primitive streak is also designated the primitive 

 groove, its borders the primitive folds. 



FIG. 373- /, Ovum of the rabbit, after van Beneden; Z, zona pellucida; e, epiblast; h, hypoblast; u primitive 

 mouth. //, Ovum of the rabbit with the (clear) rudimentary embryo; at u the earliest formation of the 

 primitive streak (or primitive mouth) can be recognized. ///, Ebr, Rudimentary embryo from a somewhat 

 older rabbit-ovum; pr, the primitive streak, with groove. IV, Still further developed embryo (seventh day); 

 the rudimentary embryo (Ebr) exhibits above the primitive streak the first indication of the spinal furrow 

 (after Kolliker). 



The embryonal area later on loses its pear-shaped form and be- 

 comes dumbbell-shaped. The portions of the germinal vesicle adjacent 

 to the rudimentary embryo become more transparent, so that the latter 

 is surrounded by an area pellucida, about which the dark embryonal 

 spot, or opaque area, is situated. The zona pellucida now acquires 

 a villous appearance, becomes covered with a gelatinous layer and is 

 designated the primitive chorion or prochorion. 



In the dog the zona becomes covered in the uterus with a coating of mucoid 

 secretion. Bonnet was able to demonstrate that this tenacious secretion pene- 

 trates into the lumina of the glandular ducts and thus forms gelatinous filaments, 

 which formerly were erroneously looked upon as villi springing from the zona. 

 They serve as a means of attachment and of nourishment for the ovum. 



Later on a new layer of cells extends from the primitive streak 

 between the epiblast and the hypoblast, namely the mesoblast (Fig. 

 376, I), which soon advances over the region of the embryonal spot and 

 continues to grow into the germinal vesicle. Blood-vessels form, further, 

 within the mesoblast, and their area of distribution upon the germinal 



