THE ALUMNI JOURNAL. 



negro race. A fact that stands out as a 

 glaring example of the most terrible and 

 inexplorable of all laws discovered by 

 scientists, that of atavism. After the 

 penetration of spermatozoids and their 

 union with the vitellus, the appearance 

 of the vitellus undergoes a change. This 

 change consists in an enlargement of the 

 granules and their more complete separa- 

 tion from the clear substance of the vitel- 

 lus. 



Polar Globule. — The next process ob- 

 served in the ovum is the separation 

 from the vitellus of a clear, rounded 

 mass, called the polar globule. And 

 now the germinal vesicle disappears. 



The formation of the polar globule 

 takes place by the projection of a portion 

 of the vitellus, which slowly detaches it- 

 self from the yolks. A few hours later 

 the polar globule again merges into the 

 substance of the vitellus, and the latter 

 organ takes on its original form. 



Segmentation of the Vitellus. — Almost 

 immediately following the phenomena 

 just described the vitellus undergoes the 

 process of segmentation, by which it is 

 divided into a number of small cells, 

 called the blastodermic cells. 



As segmentation has been studied in 

 the inferior animals, there appears at the 

 site of the polar globule a furrow, and 

 another immediately opposite ; both 

 deepen until the vitellus is divided into 

 two globules. These globules then di- 

 vide into four, then eight, and continue 

 to subdivide until the vitellus is divided 

 into a large number of cells. 



When the cells of the blastoderm are 

 completely formed they are pushed 

 against the vitelline membrane by the 

 formation of a certain amount of liquid. 

 It is probably in this condition that the 

 ovum passes from the fallopian tube into 

 the uterus at about the eighth day after 

 fecundation. 



Primitive trace. — The cells formed by 



the segmentation of the vitellus form a 

 layer in the form of a membrane called 

 the blasto-dermic membrane. This mem- 

 brane ultimately subdivides, as I shall 

 describe in a few moments. 



Soon after the formation of the single 

 blastodemic membrane a layer of smaller 

 cells, accumulate at one place, surround- 

 ed by a dark area, this is called the em- 

 bryum spot. The portion in the centre 

 of the ;iew cells is called the primitive 

 trace. There are are only temporary 

 structures and soon disappear. 



Now a permanent double fold appears 

 with a groove between , and this ulti mately 

 closes over and forms the first evidence 

 of the spinal canal. At the same time 

 the single blastodemic membrane sep- 

 arates into layers and in profile section 

 we have the following arrangement : 



There are three membranes, the epi- 

 blast externally, the meso-blast which 

 has two layers and the hypoblast inter- 

 nally from theepiblast and external layer 

 of meso-blast we have developed the 

 bones, muscles, skin and nervous system. 

 From the hypoblast and internal layer 

 of the mesoblast are developed the digest- 

 ive tract, the heart, bloodvessels and in- 

 ternal organs generally. 

 In the mamalia a more extensive and 

 complicated process takes place, for here 

 the new being is developed in the body 

 of the mother and consequently it is 

 necessary that certain connections be- 

 tween them must take place. This is 

 obtained by the formation of the foetal 

 membranes. 



As we have seen the external covering 

 during the first stages of development of 

 the ovum is the vitelline membrane, but 

 this soon disappears and when the fecund- 

 ated ovum is received into the uterus it 

 consists only of the blastodermic cells 

 subdivided into the laj^ers described be- 

 fore. 



The blastodermic cells are, as vou 



