842 



IMPEEGNATION. 



mass then remaining in the ovule being permanent, and is called the female 

 pronudeus. The next change observed is the appearance of a zone, of 

 radial striae around the pronucleus and its migration to the centre of the egg. 

 The spermatozoon then penetrates the wall of the ovule, probably at the 

 point of the formation of the polar vesicles. The tail of the spermatozoon 

 becomes absorbed, and the head is metamorphosed into the male pronucleus. 

 From the male pronucleus a number of radiating striae are given off in all 

 directions, and it then migrates toward the female pronucleus, and afterward 

 fuses with it, forming a single or cleavage nucleus. 



784. Cleavage or segmentation of the vitellus then begins (Fig. 218), 

 by which process the nucleus thus formed divides into two parts, each taking 



FIG. 218. 



FIG. 219. 



Diagrams of the Various Stages of Cleavage of the Yolk. (After Dalton.) 



with it half of the vitelline mass. These two divide into four, and these four 

 into eight, and so on indefinitely until an agglomerate mass of nucleated 

 cells results, each of which contains a part of the cleavage nucleus. This 

 mass of cells is called the mulberry mass, and the 

 cells constituting it arrange themselves about the in- 

 terior of the zona pellucida and form the blastodermic 

 vesicle or membrane. This membrane then splits up 

 into two layers, the external and internal, a third 

 or middle layer being afterward formed between 

 them. 



Immediately after the formation of the two layers 

 of blastoderm, an opaque rounded collection of small 

 cells occurs, called the area germinativa or embryonic 

 spot. (Fig. 219.) This spot then becomes elongated, 

 and in its longitudinal axis the first trace of the em- 

 bryo appears as a faint line, termed the primitive trace, 

 this being in the midst of a clear elongated mass of 

 cells, the area pellucida, which is itself surrounded by 

 a more opaque zone. 

 785. In front of the primitive trace two folds are formed from which 

 a groove is prolonged backward in a line with the primitive trace. These 

 folds gradually extend along the entire length of the groove, and from the 

 laminae dorsales, which, by growing, project more and more above the 

 groove, and, gradually approaching each other, coalesce and enclose the 

 neural canal, which will afterward contain the cerebro-spinal axis. At 

 about the same period, corresponding to the development of the dorsal 

 laminse similar laminae are given off from the under surface of the blasto- 

 derm. These are the lamince ventrales, which, by gradually enlarging and 

 finally coalescing, enclose the abdominal cavity. Beneath the floor of the 

 groove above described a delicate, whitish collection of cells appears. This 

 is the chorda dorsalis or notochord, around which are afterward developed, 

 the bodies and processes of the vertebra. 



Impregnated Egg, with 

 Commencement of For- 

 mation of Embryo. (Af- 

 ter Dalton.) Showing the 

 area germinativa or em- 

 bryonic spot, the area 

 pellucida, and the prim- 

 itive groove or trace. 



