980 CHRONOLOGY OF HUMAN DEVELOPMENT. 



Brief mention should be made here of the formation of the fetal membranes 

 in animals, which has been followed since the time of Home, Blainville, H. 

 Milne-Edwards, Owen, and others, in the classification of mammals. 



1. The oldest mammals have no placenta or allantoic vessels at all: these 

 are the mammalia implacentalia, namely marsupials and monotremata (duck- 

 bills and echidna). In addition to a serous capsule and an amnion devoid of 

 villi, these animals have only a large yolk-sac which contains vessels, but which 

 never undergoes placental formation. The allantois remains rudimentary (in 

 the kangaroo- bear it becomes larger, and together with the yolk-sac, serves as a 

 respiratory organ). (In the oviparous monotremata the ovum develops outside 

 the maternal body.) 



2. The second group includes the mammalia placentalia. Among these: 

 (a) the mammalia nondeciduata possess only chorionic villi (supplied by the 

 allantoic vessels), which project into depressions in the uterine mucosa, from 

 which they retract during parturition (placenta diffusa, for example pachyder- 

 mata, cetacea, solidungula, camelida). The umbilical vesicle, which, at the 

 earliest period, contains vessels, subsequently undergoes a marked involution, 

 in the different groups of animals, into manifold modifications. In the ruminants 

 the large villi are arranged in groups, and they grow into the greatly hypertro- 

 phied rolls of mucosa (cotyledons) corresponding to the uterine glands, from 

 which they retract at birth. The ovum is for a long time spindle-shaped. (6) 

 The mammalia deciduata form such an intimate union between the chorionic villi 

 and the endometrium that the corresponding portion of the latter must be thrown 

 off at birth. Here, the placenta is either girdle-shaped (placenta zonaria), as 

 in carnivora, pinnipedia, elephant, hyrax; or it is disc-shaped (placenta discoidea), 

 for example, in apes, insectivora, rodents, alipeds, and edentates. 



The same placental formation as occurs in man is found in the anthropoid 

 apes, but in none of the others. 



Certain variations occur in different animals in detail with reference to the 

 formation of the fetal membranes. In rabbits the umbilical vesicle also is greatly 

 expanded, and the large omphalomesenteric vessels participate in the formation 

 of the placenta through the development of a yolk-sac placenta. Also in guinea- 

 pigs (which, remarkably, have the three germinal layers in a reverse order, the 

 epiblast within, so that in the folding-off of the embryo, the latter sinks into 

 the interior of the umbilical vesicle) the omphalomesenteric vessels play a prom- 

 inent part in the formation of the placenta. In some carnivora (cats), the um- 

 bilical vesicle is provided with vessels until the time of birth. It is to be noted, 

 finally, that, in the uterus of the smooth shark (mustela laevis) a yolk-sac placenta 

 is formed. 



CHRONOLOGY OF HUMAN DEVELOPMENT. FETAL MOVE- 

 MENTS. 



Development in the First Month. The youngest ovum is described by Hub. 

 Peters. It had a diameter of i. 6X0. 8X0. 9 mm., and consisted of a vesicle about 

 three days old. The small villi had already a covering of two layers of cells. 

 It is the only observed ovum aroundwhich the capsular deciduahad not yet closed. 

 At the point of attachment of the ovum to the uterine mucosa, there were large 

 blood-lacunas, in which the ovum appeared embedded. 



Ova of from six to eight days have been described by Merttens and Siegen- 

 beck van Heukelom. They possess small short villi covered by a double layer 

 of cells; an outer layer of large cells (syncytium), derived from the uterine epi- 

 thelium, and a subjacent layer, formed from the ectoderm cellular layer of Lang- 

 hans. An ovum seven or eight days old was 3.7 X 4 mm. in size; the villi already 

 .mail ramifications. From the twelfth to the thirteenth day: (5.5 mm. and 

 3.3 mm. in diameter) there exists a simple germinal vesicle, which, at one point, 

 contains the germinal area consisting of two layers of cells. Ova from the 

 nrteenth to the sixteenth day have a diameter of from 5 to 6 mm., with simple 

 cylindrical villi, or are provided with bulbous processes from the base to the apex. 

 I he youngest ovum of Allen Thomson he estimated to be about fifteen days old. 

 t was 13.2 mm. long, oval, and provided with villi; the germinal vesicle was 

 2.2 mm. (abnormally small), the rudimentary embryo 2.2 mm. with a spinal furrow 

 and spinal ridges, projecting beyond the vesicle at each extremity; the rudimen- 

 ,ary heart was present (and the amnion?). A somewhat older ovum studied bv 

 the same investigator was 6.6 mm. long, with short, thin villi, and a large germinal 



