46 THE BIOLOGY OF TWINS 



stages very little increase in the actual mass of tissue 

 has taken place. The egg has merely increased in 

 diameter owing to the accumulation of fluid in the 

 trophoblast cavity. 



Simultaneously with the development of the Trager 

 and its invasion of the uterine mucosa there begins 

 a period of rapid cellular proHferation and consequent 

 tissue growth, evidences of which have already been 

 noted in Fig. ii. At the stage shown in Fig. 12 the 

 Trager has deeply invaded the maternal mucosa by a 

 process analogous to that observed in invading tumors. 

 It has been deemed advisable to omit from the drawings 

 that part of the Trager that has penetrated the maternal 

 tissues and to show by broken cellular contours the 

 points {tr) where the vesicle has been severed from its 

 nutritive connection with the mother. A vesicle Hke that 

 shown in Fig. 12 is more than twice as great in diameter 

 as that shown in Fig. 11, and the increase in size is due 

 in part to the marked enlargement of the extra-embryonic 

 cavity, in part to the expansion of the cavity of the 

 ectodermic vesicle, which is now a true amniotic cavity 

 {am c) . The subspherical ectodermic vesicle has thinned 

 , out on the side toward the extra-embryonic cavity to 

 form the ectodermal component of the amnion {am). 

 The embryonic ectoderm is now a vesicular mass of cells 

 somewhat elongated in the bilateral axis of the uterus and 

 with anterior or apical end at X. The embryo is really 

 a gastrula turned inside out, and hence the process 

 deserves the name "germ layer inversion." If the 

 amnion were cut and the germ-layers were reinverted, 

 we should get a normal gastrula with the apical point 

 up and the basal parts down. It should be emphasized 



